It could be fun to be able to convert castle and be able to train the UU of the other civ (in this castle you won’t be abble to train your UU except if you have the same civ…).
It won’t be used in 1vs1 or only in exceptionnal cases. It will be use in diplomacy map. For example exchange castles or trade a castle for ressources.
It will be a bit like mayan converting a stable and be able to train xolot warior.
It will increase the possibility of the game and will be really fun in diplomatic game or in game you play with your brothers or close friends who are casual.
I admit that my example isn’t the best.
20y ago, I try many things to be able to train other UU. Build a large number of monk to convert castle. In map editor, add caslte to gaya and select the civ of gaya. But nothing worked. A big disapointement and frustration