In my current game, I have had plenty of wars, so I made a Citadel with one of my generals. Many turns later, I wanted to construct another one, but the general just sits there, and acts like a worker, "waiting" for the Citadel to complete. It says it will take 4 turns (what?), but nothing happens. I make sure that I'm constructing the Citadel 1 tile outside my borders, so it's not that I'm too far from my own lands. Reloading the game etc. has no effect.