Why RimWorld sold more than 1 million copies

RimWorld is a very popular management game where you run a sci-fi colony. On Steam, the game is described as:

A sci-fi colony sim driven by an intelligent AI storyteller. Inspired by Dwarf Fortress and Firefly. Generates stories by simulating psychology, ecology, gunplay, melee combat, climate, biomes, diplomacy, interpersonal relationships, art, medicine, trade, and more.  

The game has been in development since 2013 after a successful Kickstarter campaign, and was finally released in 2018. It has sold more than a million copies. At first glimpse, the game doesn't look like much because it's a 2d game and has no fancy photo-realistic 3d graphics. Why then is it so popular?

RimWorld is an indie-game by Tynan Sylvester. If you play the game you can see he's doing everything he can to market himself: The sub-title at the start menu says "A story generator by Tynan Sylvester," and from the start menu he's linking his Twitter-account and a book he wrote about game development: Designing Games: A Guide to Engineering Experiences. It was published before RimWorld so you will not learn how he designed that particular game, but he used knowledge from the book when designing the game. He has also talked about the game at GDC 2017, which was published on YouTube not long ago: RimWorld: Contrarian, Ridiculous, and Impossible Game Design Methods. If you watch the GDC talk you'll see he had help from multiple people for audio, visuals, and programming, so he didn't make RimWorld on his own.

Lessons learned:

• It combines manufacturing, survival, and social. According to the Paradox Podcast, RimWorld is a fun game because it combines the games Factorial (manufacturing), Don't Starve (survival), and The Sims (social). In RimWorld you need to manufacture (grow and cook) food to survive, research new technologies to eventually travel home, and you also need to form relationships with neighbors, who for some reason are on the same planet, as well as form social relationships within the group, and build yourself a cozy place to live. 
• RimWorld is a story generator. Tynan Sylvester explained in the GDC talk how he was inspired by the game Dwarf Fortress - not the graphics, but how Dwarf Fortress can create stories. The player interacts with the game and stories emerge from that interaction. Some players will lose the game, which may be a story because some stories end in failures. This is also why there are no Steam achievements, which would break the story. The thing is that RimWorld hasn't come up with a story generating AI because it's using a concept called apophenia, which is a term that refers to the human tendency to detect patterns in randomness. What's happening is that the game "sends entirely random events the player’s way, and yet it’s easy to ascribe meaning to them," but people don't see it as random events. The characters have no facial expressions so you can't see what they feel, so you have to imagine it based on the situation. In the developer's book he wrote that "Medieval [another game] doesn't really track human courage or familial affection. But the human mind sees stories anyway, given the slightest of suggestions." But making a simple system can also result in trouble as real social relationships are complicated. For example, this article has found that "Men [in RimWorld] are about eight times as likely as women to try and start a romance," so players can become upset at the game if they have other values. So if you implement a simple social system, be careful!
• All characters in RimWorld feel special to you. In RimWorld you have just a few characters: you start with between one and three, and end up with maybe eight to twelve. Compare this with other games where one character is killed and you don't really care about it. If a character in RimWorld dies, you have the option to build a grave and the other characters will be sad. If a person is wounded, you have to make the decision to treat the wounded or let the character die. If you treat the wounded, the character will use resources like food and medicine while the character is useless because the character can't work, so the other characters have to work harder.  
• The graphics is not distracting the player. When I first looked at the game I thought it was made by the developers behind another management game: Prison Architect. But it's not. Both games were revealed at the same time so I'm not sure who was inspired by who? Basically the art style is a top-down 2d game and the characters are leg-less blobs (but you can still give them pants). The game is made with Unity (and Photoshop to make the art) and is using the built-in font style that comes with Unity. The UI boxes are just one color with a frame in another color. So it has no fancy realistic 3d-graphics, but the graphics is not ugly so the graphics serves the purpose: you can always see what's going on - even if you have zoomed out. Fancy visuals may sometimes distract the player from the story when many things is happening on the screen. Also making fancy graphics takes a lot of time - and time is also a limit so sometimes it's better to focus on gameplay - there are no animations in RimWorld. Moreover, many people don't have the latest computer so I suspect one reason the game is popular is because you don't need to have the latest computer to play it. Neither does it take a long time to download the game because it's just 500 MB. To make it as simple ass possible for the player to see what's going on, RimWorld is cheating: some objects that should be behind a tree are on the top of the tree:

• Don't throw away any ideas. While developing RimWorld, Tynan Sylvester was saving all ideas he ever discovered in a document because ideas are easy to forget. If the idea doesn't fit into your current plan, write it down because the plan may change. He then moved these idea to another document which was the more short-term to-do list. 
• It's easy to learn RimWorld but difficult to master. A saying in the game industry is that a game should be "easy to learn and difficult to master." For example, the game pubg is very easy to learn, but it's really difficult to get a "chicken dinner" (win the game). RimWorld has a short tutorial which covers the basics but nothing more. To learn the rest, the game has smaller hints that you can click through. Making a tutorial is a dangerous balancing act - you can't give the player too much information because then it becomes boring to play because it's fun to figure out some stuff on your own, so the game has a wiki as backup if you are stuck.
• The game keeps you in the flow channel. The flow channel is an important topic in game development, and it says that to enjoy a game, the tasks have to match the difficulty. If the game is too easy, you will be bored, and if it's too difficult you will not enjoy playing the game - so the game has to keep you in a channel where you will always meet challenges that matches your experience. For example, if you play a shooter: start with good gun which will become boring after a while → add harder enemies which will become boring because it's too difficult → add better gun → add better enemies → and so on. In the beginning of RimWorld, you have to start food production and build a basic shelter. But as the game progresses, and as you learn more, it becomes more and more difficult because some characters will be wounded and can't contribute, so you have to prioritize more and more: should you harvest crops or work on researching new technologies? When you have learned the basics you will realize that you don't have enough resources on the map you start, so you have to form caravans and travel the world to find more resources. So RimWorld becomes more and more difficult to play as you become more and more skilled. Yes, there are dull moments where no disasters (raids, solar storms, battery explosions, etc) are trying to kill you, but you know that you need those pause moments to prepare so you can survive the next disaster. 
• The weather system is affecting the game. In an earlier article I was complaining about the weather system in the game Parcitect, which didn't affect gameplay in a noticeable way. But the weather system in RimWorld is really out to get you. If there's a lightning storm, forest fires can start and burn down your buildings and you have to hurry out with your colony and extinguish the fire. If it's too hot outside, your food will go bad and you have to build a cooler to freeze the food. Sometimes a sun storm hits you, which temporarily malfunctions the cooler, and you will be really worried that your precious food will turn bad.  
• Making a game moddable while developing it is difficult. One great way to add more content to your game without spending time nor money is to let other people add it for you by making your game work with mods. The problem is that the code will change rapidly while developing the game, so a better way is to let people add mods when the game has stabilized, or people have to update the mods all the time which is really time consuming. This is what happened with RimWorld: "many creators of bigger mods have given up trying to update their mods every time a new update comes out." 
• You are directing the characters - you don't control them like drones. It would have been possible to increase the control of the characters. For example, if two characters don't like each other, then separate them. But that would also have been boring and made the interface even more complicated: "The goal of the game isn't for the player to be able to exactly restrict every colonist to do the exact optimal thing. It's to give a certain level of generalized control, and make the player accept some narratively interesting but sub-optimal interactions that might happen within that."
• Addictive games have responsibilities. RimWorld is an addictive game and to help people not forget about the time in the game, you have the option to enable a clock. I originally thought it would tell the time in the game because RimWorld has a night-day cycle, but it's telling the time in your time so you don't forget an important meeting.
• Procedural terrain increases replayability if gameplay changes. The basic idea behind the game is to build a spaceship and fly home. This could become repetitive. The goal would have felt repetitive if you had done it over and over again in the same terrain. But RimWorld has procedural terrain and different types of terrain: mountain, forest, jungle, desert, etc, so after finishing the game you ask yourself "Can I also do it in the jungle terrain?" But it's important that the terrain is different and not just procedural. I played the game Bad North which has procedural islands but the game feels the same after a while even though each island is different - but not different enough to change gameplay. In RimWorld, if you first play in a forest area and then play in the desert you need another tactic because you can't find trees easily in the desert. 
• Gameplay is more important than realism. Why are there no toilets in the game? Would have been a great fertilizer? Why is there no water management? There are smaller lakes with water in the game, but for some reason you can cook food without ever picking up water? It first felt like a missed opportunity before I read a Reddit AMA. Water wasn't added because "colonists have a lot of work to do so the needs have to be a lot more lightweight," meaning that the colonists won't have time to eat food, wash themselves, and do some work because each day in the game is short. Some in the same thread argues they really wanted water management in the game. It's the same with ammunition - each bow has an infinite amount of arrows, so making ammunition would maybe take too much time? In the GDC talk, Tynan Sylvester explained that he simplified everything that wouldn't contribute to the story. It's not realistic to dig metal directly from metal-rocks and then use it directly to build a metal-chair, but adding parts between wouldn't have improved the game. 

What could have been improved:

• Annoying UI. If you read through the RimWorld reviews on Steam, some complain the UI is not easy to understand. This is true: everything about the game is minimalistic and the UI is grouped in the bottom of the screen. 
• A better way would maybe be to group the UI with icons on different parts of the screens, like Cities: Skylines or Parkitect. Text is sometimes better to use than icons, but after a while you learn what the icons mean, so it doesn't make a difference in a game. 
• Some UI elements, such as the UI showing which zone types you have placed in the game is not visible enough - they blend too much with the background.  
• Why are orders in the architect menu? 
• Why are build roof in the zone menu and not structure menu?
• Why can't you see, when selecting which tasks a character should prioritize to do, if the character can increase its skills more than other characters when working with that task. You can only see current skills - not the "passion," so you have to click back and forth between the character's bio UI and work priorities UI.
• When building for example a chair, you can change the material of the chair from wood to steel or whatever, and it took some time to find this menu because it's just a small triangle in the top-right corner of the chair icon. It should have been included in the tutorial because it's not obvious you can click on the triangle to change material. 

• When manufacturing, everything is basically text, so sometimes it can be hard to figure out what you are manufacturing. A better idea would have been to use text in combination with an image, so you don't have to tab out and google what it is. For example, what is an "ikwa"? The description is empty (this might be a bug because the ikwa has a description in another place in the game), so you have to tab out, google, and the first result is the RimWorld wiki, which has an image of a spear.  

• Odd behaviors. Each character has some characteristics, such as being able to treat wounded, or grow plants. One of my characters could for some reason not carry items, such as transporting food from the growing area to the storage area. But on the other hand the character could cook food, so you could see him go to the storage area and pick up food and carry it to the cooking station. So why couldn't the same character carry food to the storage area? I could have accepted that some characters can't do high-tech research, but not being able to carry potatoes from the field to the fridge when it clearly can carry potatoes from the fridge to the cooking station?
• Difficult to prioritize tasks. A management game is all about prioritizing tasks so it has to be as simple as possible. Currently, you can tell a character to prioritize a task, such as cooking before cleaning. If you want to prioritize a certain tree to be cut you have to select a character and then the tree. But what if you want the tree to be cut by just one of the characters without you knowing which - you just want the first available character. This was solved in the game Oxygen not included, which is similar to RimWorld but you are mining up/down below the ground and not aobe the ground. What Oxygen not included did was that you can click on an object and set a priority to that object. So if you want a tree to be cut by first available worker, you increase the priority of that tree. In the game Settlers, you can prioritize which items to be hauled before another item, which is impossible in RimWorld. It's really annoying when you see a colonist haul a single joint while leaving the precious food to go bad in the rain. I think there's some built-in prioritization because you can see them build defense structures before all other structures, but I haven't found a way to change this prioritization.    
• Odd parameter values. This is the same problem as Parkitect has: it's difficult to determine what makes a room beauty. In RimWorld you can place floors, sculptures, furniture, flowers, etc to make a room "beauty." But even though you have placed many of these items the characters are still complaining, and if you google you see many other people are confused by the same topic. Perhaps a better idea would have been to use the same system as in Cities: Skylines where you see clearly on a color-map where you need to add more parks. Yes, RimWorld has a beauty display, but it's confusing. For example, why has one part of the drawer a beauty of 1 and the other part a beauty of 3?

• The storage area need a search function. For each storage area you can set with a check-mark if something should be stored in it, such as food but no weapons. But sometimes it's difficult to find what you are looking for, so it would have been easier if you could search for it. It has a hierarchy, but sometimes you have no idea within which group something belongs. For example, where's the Luciferum checkbox? It's below Manufactured → Drugs. If you click on the Luciferum description it doesn't say anything about that it's "manufactured." So if the search function is too difficult to implement, then it should say in the description where the item is in the hierarchy.