It's a big procedural crafting game. The long-term goal is to make a Civilization-like game set in a Minecraft-like world, with really good AI. It's also a testbed for a bunch of ideas I've developed about massive, virtualized simulation. What that means is that you could in principle have thousands of cities with millions of individual inhabitants going about their business. But it's sort of analogous to lazy evaluation in that things are only computed if they would be perceived by the player, or need to be consistent with past information the player already knows.