How compatible is Freeciv with the commercial Civilization games? Edit
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.
This is why Freeciv comes with three game configurations (rulesets): the civ1 and civ2 modpacks implement game rules, elements, and features that bring them as close as possible to Civilization I and Civilization II respectively, while the classic ruleset tries to reflect the most popular settings among Freeciv players. Unimplemented Civilization I and II features are mainly those that would have little or no benefit in multiplayer mode, and nobody is working on closing this gap.
Relevant discussions on the freeciv-dev mailing list:
So the goal of compatibility is mainly used as a limiting factor in development:
when a new feature is added to Freeciv that makes gameplay different, it is always implemented in such a way that the "transitional" behaviour remains available as an option.