My process of encounter design process in freemium mobile game

As an aspiring designer, my first design task in my first job is to create encounters for PVE levels in our F2P mobile strategy game, League of War: Mercenaries. The primary goal is to make these levels interesting. PVE experience exists in almost all F2P mobile games as the first and basic part players interact with. And they are kind of similar in terms of design philosophy — matching player growth with increasing difficulty to keep players in flow. However, for low-level design practices for this task, there are a lot of uncertainties for me. And I am inexperienced on it. So the first thing I am trying to do is search on the Internet for some pioneer’s suggestions because a lot of designers must have done similar tasks before. Disappointingly, I can’t find any article or discussion in this topic. Thus, I think it is great to write down and share my personal journey on this design task. Hopefully, other designers will be able to view my experience as a reference and improve their process of creating F2P PVE levels and share them again.

Design Goal

To be specific, my goal is to make levels “interesting”, which implies I should focus on the experience instead of the economy. So I would expect my levels to:Introduce in-game elements and show their unique mechanics and features.

  1. Introduce in-game elements and show their unique mechanics and features.
  2. Contribute to the sense of the virtual world in the game.
  3. Bring joy and happiness to players.

The game I am working on is about wars between mercenaries, the gameplay is sort of similar with Clash Royale with more strategic units options and in-depth elements like weather and defensive buildings. To be specific on this game, the first two goals can be translated as:

  1. Introduce various combat units and buildings. Show their relationship of supporting or countering each other.
  2. Create the sense of mercenaries war.

Emotional experience

Emotional arc is always one of the most fundamental concern when doing an experience-focused design. From my perspective, the arc is similar in most F2P PVE experience, described as “Confusion – Understanding – Insight – Expectation – Surprise (cycling from insight to surprise)”. Players always start with confusion when they interact with new contents. One of the design goals is to make players understand the mechanics behind new contents as quickly as possible. Moreover, establishing their cognition around new contents and giving them insights of how to play with it. With the repetition of these contents, players should gradually form expectations about what is coming next. At that moment, something fresh, a surprise, either like a new element or a new pattern will add great spices to the game.

Expected emotional arc
Expected emotional arc.

Available Resource

The above is more like the ideology. For practical purpose, I also have to understand what are my resource to use and what is my constraints before starting. In this game, enemy units levels, environment, and loots for each level is already set. For level structures, there are 21 zones. Each zone has three difficulties. And each difficulty has 8 to 12 levels. Each zone has different enemy commander figure and weather environment. In sum, there are 600+ levels waiting for me to feed. I should not make any change on these during the design unless necessary. Instead, my design focus is on structuring and tuning enemy units and buildings combinations for each level.

Available units and levels requires design.
Available units and levels requires design.

Learn in-game elements

My first step into this design task is playing the game. I play the game with randomized enemy units and buildings to be familiar with elements I am going to deal with. During the play, I took some notes and looked into some units details. This step serves to provide me with a general picture of the game and elements in it. Here are some examples:

  • “The four units combinations in each level is highly flexible.”
  • “The similarity among units of the same type makes them very hard to be recognized differently when placed together in one same level.”
  • “The UI design places units in the first place, but buildings and weather environment are in the second.”
  • “Buildings change the pacing and experience of the play a lot.”

There are more notes about very specific units and buildings, I won’t list all of them here.

A glance at the UI in game.
A glance at the UI in game.

Drafting principles

For the next, I will start my design attempts by drafting out principles for this design. Since principles are decisive, it is a very important step in the whole process. Be very careful and thoughtful. Even through multiple iterations, the general direction of the design set by principles is hard to change and it determines how your design evolves a lot. Based on my basic understanding of the game and design visions from our lead, these are my initial principles:

  1. Repetition in adjacent levels helps players be familiar with new elements.
  2. Contrast may stress a certain unit among the four units group in each level.
  3. Patterns through levels should be used to help player set up expectations for following levels and lower their cost of cognitive learning.
  4. Sometimes, apply some rarely used combinations to create surprise.

Due to the fact that I am not 100% understanding the differences between buildings and units, assuming these are principles applied to both (which is no longer true in the later phase with my understanding of the game growing).

Enumerations on potential design

After having these principles, I do not start creating levels directly according to the principles. Instead, I start to lay out all potential level design based on the above principles. For each potential design pattern, I only repeat it once or twice before moving to the next potential pattern. I call this method as “enumerations with increasing factors”. For example, at the beginning, I only used 6 different units to lay out different unit combination patterns:

  1. Fix one strong unit and switch the other three units for each adjacent level;
  2. Switch one unit in different slots for each adjacent level;
  3. Keep two units fixed (strongest and weakest), and cycling the rest of them.

After enumerating all potential design patterns from principle #1, I add new units into the levels and trying out different ways to introducing them as principle #2, followed by principle #3 then buildings. The fact shows it is very efficient that even enumerating all these design patterns I only use less than one-third of the levels I need to actually design finally.

In summary, by enumerations with increasing factors, I want to answer two general questions:

  1. What are the best design patterns to deliver a certain principle?What are the best design patterns to resonate among different principles?
  2. What are the best design patterns to resonate among different principles?

Modify principles and design patterns

With these precious enumerations, the next step is testing and learning from them. I find this step very stressful but fruitful meanwhile. Even the amount of levels is relatively small, to make fully use of them, I have to divide my attention into two parts and keep switching back and forth. First, I need to recall what are my expected goals for the level. Meanwhile, I also need to touch the actual experience as objectively as possible to feel whether the design truly delivers the expected goals or not. Moreover, it is better to figure out reasons why these designs work or not. The followings are selected notes:

  • “Be familiar and understand one new unit need about five times repetitions.”
  • “Introducing two fresh units at same time overwhelms players.”
  • “Zone transition is a great moment for pace changing. Design should collaborate with it. Create something different between zones and some patterns within each zone.”
  • “Unified colors or defensive buildings (counter a certain type of unit) is very painful to players depend on the certain unit a lot.”
  • “It is very neglectful for buildings and weather environment. So they require higher repetition than combat units to be noticed by players.”

After the intensive test, I gradually develop a deeper sense of all in-game elements and finally come up with the final design principles:

  1. For each zone, there should be an ambiguous story in mind, such as the commander of this zone loves tank.
  2. Consider each zone as a block for design. Make levels in adjacent zones different and levels within a zone have patterns.
  3. Introducing one to three units for each zone according to its length and the situation.
  4. Besides of the new units, choose other units from a preset library, “Common Units”, and use them repetitively to make sure new units are stressed by contrast.
  5. It is a reference pattern in a zone:
    • New, Common A, Common A, Common B
    • Same New, Common A, Common A+, Common B
    • Same New, Common A, Common A+, Common C
    • Same New, Common D, Common A+, Common C
    • Same New, Same New (Different Color), Common A+/D, Common C
  6. Insert some irregular zones occasionally.
  7. Unify weather and buildings based on zones to reduce distraction from them.
  8. There are mainly three different types of buildings: Early-phase attack, purely defensive, and unit enhancement.
An example encounter pattern.
An example encounter pattern.

Now let’s go back to see whether these principles deliver the intended emotional arc. The repetition among the same zone stress out the new primary unit to push players to notice its unique features and gain insights on it. Zone as a basic repetitive block help players to form patterns through playing, which greatly reduces their cost of conversion from confusion to understanding and also builds up expectations. The transition between zones will bring player the surprise of new units.

From problem-solving perspective, the above design principles make players project their attention on the fresh units. The levels enjoy diversity and regularity based on zones, which is make sense for our real world. Moreover, a great pacing of “expectation – surprise” cycling are conducted by encounters in zones. And the unexpected irregular zones add an extra layer of surprise to the game.

Arrange resource & finalize design

Once I am satisfied with the principles, the rest of works are relatively simple and repetitive. First, list all units and choose one-third of the units as “Common Library”. The rest of them go as primary units. Then arrange these units to all levels accordingly with one to three primary units and 3 to 5 common units for each zone. Finally, add building combinations into these zones. That is the end of the design.


Since the design have not been put into release yet nor serious player test, it is hard for me to conclude. However, besides of the process, following points are meaningful to share as well:
Be brave on using binary tree search (It is not the algorithm)! When tuning a value, the intuitive way is to start with you intuition and gradually change the value into the right one by iterations. When I am trying to figure out what is a good block size (units are repetitive in the same block) for this design, I start with three level for each block. It feels messy. So I increase the size to five, then eight, then finally realize the zone (about ten levels) can be a block. If using the binary search, I should start with my intuition, three, then adjust with a big step like 20. Each time, adjust the step with half of the previous step either forward or backward. It is more efficient on finding the right number. Moreover, it is easy to be dumb if you adjust the value with minor changes, binary search avoids it as well.

Bottom-up during exploration. Usually, I start with digging into the game to learn these elements through playing instead of proposing high-level framework. A lot of design principles come from a deep understanding of the elements. In my case, the understanding of UI confirms units are the primary design focus then generates the principle about different repetition frequency for the elements.

Top-down during design production. However, it is very important to have a big picture in mind and follow it strictly when doing the final design. It ensures the design will not run off the rail and lose original intentions. One of my total failure iteration happens when I try to arrange all units into these levels without assigning a set of units to each zone. It turns out to be a complete mess that I use up too many units at the beginning to have enough units in the later levels. And I even lose track which units have been used.

Efficient tools matter. (And they do not have to come from engineers.) The first thing to avoid when doing design is distractions. A lot of subtle things need 100% attention to capturing. Generally, I see three main distractions during design: personal mood, the cost of acquiring information, the cost of iteration. The first one depends on designer him/herself. Tools help to lower the last two distractions a lot. Before I start to tune the units combinations, I use Spreadsheet formula and conditional formatting to extract the most important information of the unit and present them in a straightforward way. It greatly lowers my cost of acquiring this information and speed up the whole process.

Efficient tool to retrieve information.
Efficient tool to retrieve information.

Postmortem for Vocal Power (Ludum Dare 32)

Vocal Power is created in Ludum Dare 32 Compo in 48 hours. The theme is “An unconventional weapon”. The game is based on an innovative concept of speech simulation. The art of speech is the powerful weapon in daily life. Vocal Power enjoys minimized control. Gameplay uses one button. By holding and releasing SPACE at certain times, the character makes a speech with different pace and interacts with audience occasionally.

Vocal Power in Ludum Dare 32



Based on the theme, “An unconventional weapon”, I propose three weapons: money, grid-based card, passion. Since I want to try some emotional ideas. I focused more on “Passion”. Gradually, I develop an idea of using passion for handling different situations. After reviewing the proposal, I thought passion might be too board, so I change it to speech and propose different situations which require speech skills. And the core gameplay is spread your voice to public by a hold/release mechanic.

I open my first bag of lenses… The concept is good because:

  1. Speech simulation is new, rare games dabble in this.
  2. Players will curious about how to do a speech and what are the new elements in the later games.
  3. The core gameplay control is simple and engaging. With only a button, players have bunch of interactions.
  4. Situations/difficulties are in daily life. These evoke emotions when playing.
  5. Audio and visual FX to make situations realistic will be important.

Bags of Lens

I decide to try using Jesse’s Lens card (The Art of Game Design, A Deck of Lenses) during creating. Kind of randomly and intentionally, I pick 9 cards into 3 bags for three cards of each bag. Planning to open them and think about it at three different times, before developing, after gameplay, in postmortem. The lenses are: Emotion, Curiosity, Novelty; Meaningful choices, Emergence, Judgement; Pleasure, Flow, Unification.

9 Lenses in 3 Bags
9 Lenses in 3 Bags


Tools: Unity 4, Photoshop, Illustrator, sfxr, Aduacity.

I start with coding following features:

  1. Speak UI. It is the core gameplay of Vocal Power.
  2. Wave spread model. How to display the actual influence of your voice is extremely important in the game.
  3. Different scenes. Public speech, party, classroom, home… Scenes inject emotions.

It takes my whole first day and night to finish these features. Then I open my second bag of lenses… When making the game I should be aware of:

  1. Provides meaningful choices. It is provided in two different levels. In core gameplay, when you speak (HOLD SPACE), you can decide to speak as long as possible to spread it further or to stop at certain positions to interact with audiences. In the general map, players can go different place to practice different skills in speech.
  2. Provide a map for players can provide players more behaviors for emergence.
  3. Judgement is in speech and the whole game. In the speech, interact with audience on time help a lot. In whole game, a timer measures how quick the player finish the whole game.

Based on these, I need to work on the following features to make it playable:

  1. A map connects all the situations.
  2. A skill system provide more long-term strategy and options for players to choose.
  3. UI to deliver the system.
  4. Basic SFX
  5. Textures for objects (Fail to do)
  6. Adjust difficulty of levels.
  7. Hint texts and tutorials
  8. Promotion art

It is a lot of work to do. Fortunately, with a 5 hours sleep, I am very effective and finish almost all of them in the second day. Actually, I spend very limited time on art and audio, which make the whole game more like a prototype.


It is time to open the third bag of lenses! Based on them, I would say the following of Vocal Power:

  1. Contents of the game is concrete and tied together. Situations, questions, like/hate, influence waves, etc… Everything makes sense. These make sure players can understand this weird concept without a heavy tutorial.
  2. Pleasure is tied with emotions. When situations evoking emotions from players real life experience, to solve the situations in the game will be fun to them.
  3. Flow. I have to admit, I forget to be aware of it when developing the levels. Even the difficulty is gradually increased in levels. The curve is too flat.

What went right?

  1. Concrete theme and SFX help a lot on players’ understanding.
  2. Scenes and map inject emotions.
  3. Cards of lenses are great reminders and checkpoints during developing.

What went wrong?

  1. The scope might be too big.
  2. I need more unity experience to learn how to use efficient tools.
  3. A more fast prototype is required to adjust the vision in a more early phase.

The Iceberg of Tutorial

The tutorial of a game is always considered as the gate of a game. A beautiful and opened gate is easy to attract players to come in and stay. On the contrast, a shabby and closed gate are easier to be ignored. The function of the tutorial as a gate is to teach players how to play. Generally, there are following ways to teach:

  1. A standalone level/chapter
  2. Behavior-related tips
  3. Randomized tips (always during loading)

In classical PC and console game, the cost of start playing a game is high. So if players enter the game, they will actually play with it for a while. However, the cost of mobile games are relatively low. So when players enter the game, they are still likely to abandon it. With the fast development of digital retailing, such as Steam, even the transition cost of PC game is already lowered. Therefore, another function for the tutorial is persuading players to stay. The followings can be a way to achieve or enhance this:

  1. Already start during tutorial
  2. Show potentials
  3. Give rewards
  4. Make it fun!

In a more deeper level, I believe its existence in my “subconsciousness”. The tutorial forms your mental process structure about this game. Just like the first impression of a person is really important even if it is hard to clarify what is the importance. If you meet the person in a professional situation, you will consider him/her in a professional way and focus on behaviors and competencies. If you meet the person in a casual situation, his/her sense of humor and habitats may be more important. In tutorial of games, think about the following questions:

  1. Do you want players to take the game seriously or casually?
  2. Which part of the game do you expect players to focus?
  3. Do you put passions when making the tutorial? (Players can feel it)

In sum, see the following figure.


The Iceberg of Tutorial
The Iceberg of Tutorial


“Currency” in game balancing



The definition of “Currency” in Collins is “2. general acceptance or circulation; prevalence”. Currency in the game means the measurement and representative to all different elements in the game. Currency which is defined by the designers can help a lot on prototyping the game system.

In Prototype

In this phase, one of the important work is to create contents such as skills, weapons, and enemies. With a currency, creating similar elements with variations are much easier.  Moreover, having a currency makes the whole system more clear which facilitate divergent thoughts.

Case 1 – Hearthstone

Bloodefen Raptor
River Crocolisk
River Crocolisk

The critical principle behind currency is the relationship between the currency and any other elements. There is an obvious currency in Hearthstone, the mana crystal.

  1. 1 Extra Card = 1 Mana;
  2. 1 Attack / Health = 0.5 Mana;
  3. Silent = 1 Mana; Taunt = 0.5 Mana;
  4. 1 Hit Point = 0.33 Mana;
  5. Hero Special Card = 1 Mana;
  6. 1 instant damage = 1 Mana.

Windfury, Charge and Divine Shield are more complex. They highly depend on the attack or the health of the minion. Based on this, creating various and balanced cards are much more fluent. At the early stage of Hearthstone, most of the cards are highly restricted by this currency. For instance, the Bloodfen Raptor and River Crocolisk are 2 Mana cost, so the summed attribute are 2*2 + 1 = 5 (All minions have a base of 1 attribute). Later, with more and more new features introduced into the Hearthstone, mana as a currency has more and more little sense of presence. Balancing different cards depends more on testing and statistics.

Advanced feature – Invisibility

Start with a currency and abandon it later. It is a very popular phenomenon in game design. The reason behind it is that currency has double-edges. In the prototype phase, currency is a good assistant, with its help designers can create a bunch of balanced contents effectively. However, when the prototype grows up, players expect more new things in the experience. Currency makes them think in a same and simple frame which impairs the feeling of having new things. So in refine phase, one of the important feature of the currency is invisibility.

Case 2 – Heroes of Might & Magic 3


Currency is useful in early phase of developing a game. Then it should be hided in the later phase.

Taxonomy in social F2P games



A simple idea towards competition is to judge who is the winner, which called direct competition. In this situation, players in a game compete with each other in certain resources. I can get more from yours loss. In early farm simulation F2P games, such as Happy Farm (2008), the stealing crops mechanic makes players get the exact number of crops from the others’ loss. So there is a problem, if the competition is about 2 players and players who lose will not be happy with that, only half of the players can get fun from it. Clash of Clans make a improvement on it, when you defeating the opponents, you get exact what you should get, but the opponents do not lose as much as your rewards. Instead, they just lose half or even one fourth of your rewards. It makes everyone feels better but still keeps the competitive feature.

There is another kind of competition where everyone is the winner but someone more outstanding will win more. (It is so ironic and reminds me of “All animals are equal, but some animals are more equal than others”). In a lot of F2P card games, they use this mechanic. For example, in Million Arthur, every week there will be events hosted by the publisher. Everyone takes part in the event can get reward. But if you want the awesome reward, you need to be better than others. In other words, you need more investments, including money and time.


The classical cooperation in online games is synchronous (direct) cooperation, such as World of Warcraft. Players need to get online at the same time and play together to get extra bonus. It elicits more investments to the game because every companion’s engagement facilitate your engagement. However, for this kind of cooperation, pressures come with encouragement from your companions. You are not only encourage to play with friends at the same time, but also you are required to play with friends at the same time to ensure your and their bonus. That is why I stopped playing WoW.

Of course, the other kind of cooperation is asynchronous (indirect) cooperation, which means players can give others hand without being same time in the game. For example, I can help my friends by helping gardening their farm in Hayday. Players are not required to playing at the same totally. But players can still feel that other players exist and matter in the game.


In summary, most free-to-play games have these two kind of interactions between players because they are so fundamental. The level of these interactions really matters the target customers of the game. Direct means more focused and heavy players. Otherwise, the counterpart is relaxed and light players. Competition indicates male and strategy players. Cooperation indicates female and simulation players.

Taxonomy in Social F2P


I killed an old man in This War of Mine

Killing spree
Killing spree

I had killed thousands of lives in virtual world. I killed them using cold weapons in Mount & Blade. I gave them head shots in Call of Duty. I teared them into pieces by my magics in World of Warcraft. I killed them so I won and got promotions, rewards, and more power. I got everything in these worlds by killing. So in these world we are killing for fun. The mechanics and systems behind these games are designed to encourage killing in certain areas and circumstances. So in these kind of games, I don’t remember how many lives taken away by me because I kill everyday and the lives are actually similar to my character.

More options
More options

Then I met sandbox games. In Skyrim, Fallout New Vegas and Watch Dog, mechanics and systems provide more variations on solving a problem. You can choose to kill since that is the most traditional way. But you have other options, such as bribe and persuasion. Compared to the games mentioned in the last paragraph, these games are less violent and more optional. Still, I don’t remember how many lives I killed too because killing is just one way to solve a problem (mostly a task) in the worlds. I am care about finishing a task instead of my way to finish it.

This War of Mine
This War of Mine

However, I killed an old man in This War of Mine. I killed him in a game one month ago but I still remember all the details of my behaviors and my mental process. And I still feel guilty. I Let me tell you that. My character was a cooker at that night, one of my companions was in charge of night watching at home and the other one was badly sicked in bed. I was going outside to scan for resources. I need food for my group and I need medicine to save my companion. Hospital was taken up by a group of soldiers so I had to test my lucks on an shabby building. It was said that there are foods and medicines but dangers as well so I took my pistol. Entering the building is pretty simple because it is a big building and few people lived here. I tried to be polite at beginning avoid entering the room which is taken up by others. I scanned for hours but failed to find anything useful. Even my character were murmuring that he is hungry.  So I decided to take a risk. I entered a kitchen which is obviously belong to someone else. Then I got some food successfully and no one spotted me. Since I had stolen something, then I think I should steal more since there was already a penalty for stealing. So I walked around and entered a bedroom. All I spotted was the clear icon floating on the bed which showed that there was items inside. So I clicked that and my character found medicines! Oh, I can save my companions life! I took them. Suddenly, a dialogue popped out, “Don’t take the medicine. I need them”. Then I found that there is an old man lying on the bed. Obviously, he was ill. Actually, in the game, I always try to help others. I traded my medicine to one of the son whose father is ill to save his life. I went outside to help my neighbors. But now I was taking other’s medicines, I was taking his life supplements away. Without any hesitation, I pulled out my gun and shot at his three times before he died. I run away and back to home. Finally, my companion still died in sickness because it was too severe and I was killed when suffering a robbery later.


Why I kill that old man? I keep thinking about it. Rationally, killing a non-enemy using bullets is a waste of bullets and they are precious in the game. The instant thought is that I took his medicine so he would die. I was just try to make it easier. It was nice to him. But now I can admit that I killed him because I felt guilty to steal his medicine and his existence was evidence of my guilty. So I killed him for my own relief.

Emotions in game is precious and important. So one interesting question is why I feel guilty to that exact old man instead of the former bodies on my game experience path. Personally, I feel the followings might be part of the reasons.

First, in This War of Mine, there is no exact goal for players. Players struggle with survival everyday, but the game never says that is your goal. Without a goal, players cannot blame killing to game. They have to take their own responsibility for their own behaviors.

Second, when a character kill someone his mood and his companions’ and his moods will be low for several days. Exposed to others makes players more affected by social opinions.

Third, NPC are responsive to the world. If the old man was just lying down or kept saying before I took medicine. I won’t feel he reacted to my behavior. Then I do not care about him any more.

Fourth, I died from robbery from someone else. I feel the hopelessness when suffering it.

That is all. There is no conclusion for that. Just hope I can think more about emotions in games.

Chance from Experience Perspective

Randomness is fun

Chance, or say Randomness, is important in a game. Why is is important? Because randomness means uncertainty. Uncertainty means surprise. Human love surprise. For surprise there is two scenarios. One is when you stayed at home, you wife came back and told you: “Our lottery got the first prize!” Second is when you looking at the lottery show, your lottery matched every number on the screen. Which is is more exciting to you? Personally, I prefer the second one. The different between these two is that the second one is involved with anticipation before surprise. For human beings anticipations strengthen emotions.

Critical Strike in WoW
Critical Strike in WoW

For the followings, I will talk about the chance in anticipation → surprise models in games. The most common example is critical hit. “40% chance to critical attack.” This sentence tell you the possibility of surprise. Think about this moment: your character deployed a critical attack; Blood splashed from the enemy body; Even the enemy was taken down with a bullet time. It is so exciting that I make a critical attack! I am so powerful! And then think about another moment. The boss had only 20 HP left and your attack was 15 but only had 1 HP. You pressed the attack button and begged a critical attack. Nothing special happens. The boss killed you for the next attack. Oh, @#$%. I had such a bad luck. It has been such a long time that I can’t deploy a critical attack! It ruins me. My former mistakes of wrong strategy choices were not important. It is just my bad luck. Chance involves an significant part of the above situations. For the first one chance gives you a reason to anticipate and receiving the more exciting surprise. For the second one, chance gives you a excuse to blame and to hide your own mistake. So you are always doing good, the game is always fun.

Critical Strike in DOTA 2
Critical Strike in DOTA 2

It works very well in single player experience. However, in multiplayer arena experience, such as DOTA, your good luck means the opponents bad luck. So if I lose a game simply by bad luck so obviously, then it means all my efforts and strategy seem to be not important in the game. Audience and players tend to make a conclusion that the game is judged by chance instead of ourselves’ skills, which is not fun. So there is a clever modification to the chance system, it called “Faked chance”. It means the current chance of expected effect depends on how long you do not have that effect. It makes sure that the game is in balance most of the times with a cost of surprise. Another extreme is “deploying a critical attack for every 3 hits.” It is totally anticipatable.

Crit Chance in XCOM
Crit Chance in XCOM

In summary, in single player games. Chance is a great way to generate the fun of surprise and a great way to capture the players for more times and spending on the game. In multiplayer arena games, chance is a trade-off between balance and fun of surprise.

Reference: DOTA uses “Faked Chance” for critical attack. World of Warcraft uses a lot of “anticipatable critical attack”. And I guess that XCOM uses “Faked Chance” for hitting. It means when one of your soldiers hit the enemy by chance, other soldiers’ hitting chance will be diminished.