The Citadel Building is impossible to build once you have a star fort. It should supported by a Castle OR a Star fort. You can build it with a Castle, but one a star fort replaces it (long before the castle obsolete), the citadel lists "requires castle" as a requirement but you can't build one since it is already built. Only by first deleting the Star fort with Ctrl+A was I able to build a Citadel. I believe a check might be in order for other castle dependant buildings (castle gates, bridge...).