I at first thought this was some sort of feature but now im sure its a bug. I captured Corinth about 100 turns ago when it was a fortress. When i captured it i was unable to train anything there but ballista despite the presence of a barracks, an archery range and some stables. i thought that it would change over time. now corinth is a citadel and i have built the citadel level buildings and i still cannot train any troops there. basically any castle that i capture will never produce units and is ultimately a complete waste of space but i still need a place to retrain local garrisons.