Sid Meier's Civilization VI

The environment rather than the subject. The essay's argument for choosing it is that complexity here is emergent rather than designed: simple opening decisions compound across three hundred turns into a state no player models completely, and six victory conditions mean no single objective dominates, so an agent has to work out what game it is playing.

Its second useful property is the debug port left running in the engine, which is what civ6-mcp turns into a tool interface.