Cities: Skylines is Turing Complete
(Non-clickbait title: building a 4-bit adder in Cities:Skylines.)
Cities: Skylines is a city simulation game that is complex enough to build universal logic gates in it. Using universal logic gates it is possible to construct any circuit including Turing complete machines. So, just like in Minecraft one can build a computer inside Cities: Skylines. However, it would be very complicated to build a fully fledged computer using these gates, so I will demonstrate a 4-bit adder instead. Everything is done in the vanilla version of the game, no mods or add-ons are required.
The game, just like other city builder games, requires the player to manage power and water for the city. Power plants produce electricity and require both clean water and sewage. Water towers provide clean water, sewage pipes get rid of waste — both of these require electricity. This sort of duality between sewage and water towers allows the construction of AND as well as OR gates.
You can see an AND gate built below. The 2 inputs are the power lines leading into the water tower (top) and the sewage pipe (bottom). The output is the power line attached to the power plant. Even though the inputs are zero in the screenshots, the power plant is still producing electricity — it takes a while for it to shut down even without water and waste management. The buildings are placed far apart because otherwise power would freely flow between them.
