Freeciv has been around much longer without being targeted by any legal threats that I'm aware of. There have also been commercial competitors like Activision's Civilization: Call to Power and its sequel that only had to drop "Civilization" from its naming for lack of a trademark license.
I'm not sure any of those aimed to be as direct a clone of a specific mainline Civ's rules and mechanics, but rules and mechanics are the aspects of the game that are most clearly free to copy.
I'm not sure any of those aimed to be as direct a clone of a specific mainline Civ's rules and mechanics, but rules and mechanics are the aspects of the game that are most clearly free to copy.