Freeciv was created as a multiplayer version of Civilization™ with players moving simultaneously. Rules and elements of Civilization II, and features required for single-player use, such as AI players, were added later. It is still a stated goal to let Freeciv's game engine be 100% compatible with Civilization™ I and II, but only as an option. Relevant discussions on the freeciv-dev mailing list: * in Jan, 1999 * in Jun, 1999 * in Apr, 2000 So the goal of compatibility is mainly used as a limiting factor in development: