They Are Billions and The 13 Basic Principles of Gameplay Design

They Are Billions is a strategy game where you must survive the zombie apocalypse and build up defences to protect your fledgling city. However, at the end of the match, an enormous horde of zombies will rush to your city, doing whatever they can to destroy it.

The 13 Basic Principles of Gameplay Design were created by Matt Allmer in 2009 based on the 12 Principles of Animation that he had learnt about at college. I will be using these as a framework to analyse They Are Billions.



Focal Point

The focal point of the game is defending your Command Centre from attack by the zombies. To do so effectively, players must build up their colony in size and resources. The expansion of the colony is a secondary objective in the game while defending your colony is the main focus of your activities. To unlock new defensive structures and troops, players must gather resources, while building research facilities and troop recruitment methods.


Each time you play the game, you choose how many days a particular run should last. Each run can last between eighty and one hundred and fifty days. At the end of this period of time, the final horde will attack. Throughout the entire course of the game, the player is anticipating the all-out-assault from the zombies at the end, shaping the way the player builds their colony and which troops they train.

Another way they are billions uses anticipation is by having each troop have two stances. One is on standby/navigating the map while the other is primed to attack. This way, the player can tell when their troops are ready to immediately attack, and when they are waiting to be told to attack or are in the midst of following a move or patrol order.

Announce Change

The map is filled with stray zombies which are attracted to human troops and structures by noise. Occasionally, they will wander towards your colony and attack. Separate from this, are zombie waves. These are announced to the player whenever there will be an attack along with what direction the attack will be from. The player is then given eight in-game hours to prepare their defences in the given direction before the horde makes its way through the map, picking up a few stray zombies along the way.



Believable Events and Behaviour

As you are exploring the map in They Are Billions, you will encounter zombie villages, old settlements that are overrun with zombies. These will continually spawn zombies to come and attack your colony, so it is key to take them out. Whenever you attack a building in a zombie town, more zombies will come flooding out of the building you are attacking and nearby ones to defend the town from attack. This forces the player to split their troops up and have some attack the buildings while others defend them.

This is an example of believable behaviour since when people’s homes or towns are under attack, they will attempt to defend themselves and others. Another way the game incorporates believable behaviour is when a colonist comes near to a zombie, they will run in any direction away from it screaming, as anyone would probably do.

Overlapping Events and Behaviour

Sometimes, smaller unannounced waves of zombies will attack your defences, at the points the game deems are the weakest. These are random, and come without warning from any direction, resulting in the player have to fight fire on multiple fronts.

When troops fight and kill zombies, their veteran percentage increases. Once it reaches 100% they upgrade, gaining higher damage, health and/or rate of fire stats. When playing the game, you can use this to your advantage by making death squads to run around killing zombies to become veterans or by posting new recruits on busy walls where zombies like to attack.

Sniper veteran and other stats


Since the game is in 2.5d, it doesn’t use basic physics such as gravity or drag. However, physics are implemented through the energy mechanic and the rate of fire of various troops and buildings. To expand your colony further, you must spread energy connections out beyond your base using Tesla towers. These bring more squares under your sphere of influence and are key to keeping all your defences running, as without energy any building defences shut down.

Another way energy is used is through shocking towers. These deal massive Area of Effect damage, but require a recharge time. This is cleverly shown through the coils around the top of the tower lighting up one after another. When the charge is fired sparks of energy shoot out of the tower. Other building defences also have delayed rate of fire and speed of missiles. For example, great ballistas have a medium rate of fire, but a slow travel time as they are giant mounted cross-bows. On the other hand, the upgraded versions of great ballistas – known as executors – have an extremely high rate of fire and fast travel time as they are automated gun towers.


Sound plays a vital role in They Are Billions to build the world around the player, to engage them in their surroundings and to create tension during critical events. Each of the 4 maps have their own environment sounds, to give each their own atmosphere. If you zoom into different aspects of the environment, each has their own sounds layered into the atmospheric noise. For example, if you zoom into a river or lake, the sound of running water is added to the background noise. I believe that Numantian Games spent a long time on the sound to ensure that the game felt fleshed out, and to help push across the steam punk setting of the game.

Whenever an action is completed for the colony, such as a troop is trained or a building is built, there is a short beep followed by an audio prompt of what has happened. This also occurs for attacks, and is useful for signalling events to the player when they may be focussing on something happening on the other side of the map. Without this feature, players may miss attacks, important buildings completions, and being able to organise their troops effectively.

Each type of building in the game has its own sound which is played when you click on it. This provides the player with information on what type of building they are interacting with, as well as giving the buildings life. For example, you can hear shouting from the Soldier’s Centre, laughter from the Inn and electricity zapping through Tesla Towers.

On top of each building having unique sounds, each zombie and troop type have their own voice lines, ranging from different pitched growls, to questions about when troops are going to be paid. This is the only time I feel the sound design could be detrimental to the game as the repeating voice lines from troops can become grating when you are fine-tuning their movement to route zombies a specific way. The troops voices still have use though, as when you select a group of troops, the most commonly selected troops voice line will play, giving the player a quick idea as to the make-up of the team especially if they check the icons displayed at the bottom of the screen.

Finally, the best way that They Are Billions uses sound is in the games music. Each piece in the soundtrack was recorded with the Bratislava Symphony Orchestra, giving the music a fullness which is often lost in games where digital instrument samples are used instead. The composer Nicolas Diteriks did an excellent job in creating a sense of looming danger and excitement through the music, and much of the game’s atmosphere and world-building would be lost without his booming score. The main theme is epic, using dissonance and rising chords through the brass section to create a feeling of impending doom before melting into softer strings to show hope that you may yet defeat the endless hordes. All the while, the percussionists hammer out a war beating, bringing a sense of the battle to come.

The build up to the end game is tense, with strings oscillating back and forth, trying to put your nerves on edge as you desperately prepare your final lines of defenses before the Final Swarm Theme crashes in. The way Diteriks used the brass to create scale and a sense of power gives me goosebumps whenever this theme begins. I have a personal passion for classical music, and seeing it utilised so effectively in an independent game shows how sound is crucial in creating a game that feels full of life.



They Are Billions has an interesting approach to pacing. As you can pause the game at any time, the player can take a breath and think about their next decision, in spite of their impending doom. When there are no zombies attacking, the game feels relaxed and slow, trying to lull you into a false sense of peace and security. As soon as zombies begin pounding at your defences, the game instantly feels faster. No matter the size of the zombies attack, every action you take has a sense of urgency, even if it is an unrelated building project on the other side of your colony. This is further added to through the games use of sound, and the way the brass section soars during an announced wave to inform you of the imminent danger.




As the game is in 2.5d, the developers have used a grid format to dictate where players can build, the sizes of objects, and to display areas from which buildings are taking resources from – this is key as buildings of the same type cannot draw resources from an overlapping area.

Walls are a key part of They Are Billions as they allow you to create layers of defences and minimize casualties when a zombie makes it past your outer-most defences; however there is a 2 layer limit to any wall that you build. If you attempt to build a wall 3 layers thick, the game blocks this action. I believe the game developers introduced this to prevent players from building incredibly thick walls to simply delay the zombies in one part of the map forever while they deal with an invasion elsewhere.

In They Are Billions, there are 4 possible maps, each with their individual terrain and challenges. Every map plays differently, and there are positives and negatives to each differing environment. By giving each map specific attributes, the developers can play around with the spacing of resources and obstacles to keep the game fresh as the player’s skill level increases. The maps are as follows:

  • The Dark Moorland – this map is unlocked at the start of the game, and has lots of terrain obstacles such as lakes, rivers and mesas which minimises free building space. While this can make organising your colony more challenging, it does allow for fantastic defence layering.
  • The Peaceful Lowlands – compared to The Dark Moorland, this map has a lot more building space, but is covered in sand making it difficult to build farms – a key food resource for many colonies. By changing the ground type in a lot of areas, the developers force the player to think strategically when building their colony.
  • The Frozen Highlands – the developers cleverly interpreted the theme of coldness in this map, by making all units move roughly 20% slower. This changes how easy it is for you to mobilise units to an area of you colony that is under attack and makes it feel like walls are infinitely further apart as you watch your troops trek through the snow and ice.
  • The Desolated Wasteland – the final map of the game is by far the most challenging. The map is overall open, but it has a larger number of Villages of Doom – a series of infected buildings that spit out zombies over time and must be removed before the end of the game to stand a chance at surviving. Furthermore, forest areas are few and far between, as well as green fields to build farms upon. Despite the openness of the map, it is difficult to build housing areas due to the prevalence of minerals such as iron or stone, which cannot be built upon.



Linear Design or Component Breakdown

This game has many different resources which you must balance alongside defending yourself from the oncoming zombie hordes. Each resource is interlinked with the others through the buildings, as to gather more of a resource, you need to use up your supply of a different resource. For example, stone quarries require wood to build, which need sawmills to collect that need gold for the workers. These workers need food, homes of their own, and said food and homes cost more supplies. Therefore, I believe the designers of the game used the component breakdown principle when creating They Are Billions. By identifying all the major sections of the game before starting to code it, Numantian Games ensured that all parts of the game would feel connected, and that no resource, building or battle would exist in a vacuum.




In They Are Billions you are the leader of possibly one of the last pockets of civilisation in the world, which gives context to the level of power you have over the colony and your ability to fine-tune every last detail down to the exact spot a troop will walk to. Players are able to make the game as hard as they like using the population and number of days settings before they start a new game. This affects their score modifier, which can range from 7% of their points to 440%. By giving players this level of fine control, the game developers ensure that despite creating a high skill game, even players less experience can give the game a go. More experience players often restrict themselves to create gameplay challenges such as only using the weakest class of troop, or not building one type of building that makes the end game easier.

The music in They Are Billions is critical in how the game makes the player feel, and their anticipation. So much of the game is built around the idea of the enemy are on there way and that it is inevitable that you will be attacked, that the frantic energy of the music is key in keeping the player engaged during tense sections of gameplay. It builds on the atmosphere for the player, and makes the world feel fully realised.


Communication is key in They Are Billions due to the way that priorities can quickly shift for the player due to a completed building, a random attack on your colony, or an impending zombie horde. When the player reaches a population goal, a pop up will appear to allow them to select a mayor, which is accompanied by a brief change in music and a voice alert along the lines of, “a colony needs a mayor!” To ensure that players are less likely to miss events happening across the map, alerts pop-up in the top left of the screen, as well as an announcement over what has occurred. If there are lots of alerts at once, only the first alerts voice line will play, and the rest will be notified to you using a beep. These announcements commonly include:

  • “Units under attack!”
  • “Attention: building completed.”
  • “Our defences are being attacked.”
  • “The infection is spreading across the colony.”

A lot of the game’s communication is done through sound, using announcements, changes to music and various sound effects like scraping on walls to signal to the player what is happening in the game. This was a good decision by the game designers, as when you have a large colony, you may be working on multiple projects at once, and visual alerts may not have been enough on their own for many key events.

The HUD on the bottom of the screen is where a lot of the gameplay takes place for the player. There you can see all your resources, the map, date, pause/play button and buildings you currently have researched. When you have the build menu open, anything that you currently do not have the resources for is greyed out, to indicate that it is currently unavailable for building. Also, when you have completed the research for some buildings, there are often holes left in rows of buildings, showing that there are more buildings to be unlocked through advanced research tiers.


The main appeal of They Are Billions is its unique take on the Real Time Strategy (RTS) formula by creating a crushingly difficult zombie game. They Are Billions has a target audience of experienced gamers, if you are new to the RTS genre, this is not the game to start with. By making the game so that a single mistake is potentially devastating or game-ending, the developers Numantian Games clearly aim the game at fans of this genre.

Another part of They Are Billions that makes it appealing to the player is that you play as the underdog against a far superior force throughout the game. When I play the game, I love taking out zombies with careful planning and strategy. This makes defeating the random waves and the end horde incredibly satisfying as you have beaten incredible odds.


They Are Billions cleverly utilises these principles of gameplay design to create a fun yet challenging game set in a cohesive universe. While some principles were possibly under-utilised such as physics, others were key in bringing the game together. The most important principles in the making of this game were sound, announcement of change, anticipation and communication. Without all four of these principles working together, players may have missed key events while playing that could affect their current plan and change they building strategy.



Allmer, M. (2009) The 13 Basic Principles of Gameplay Design. [website] [accessed

They Are Billions Wiki[website] [accessed 19/10/18]