Speaking of unique units, it seems to me that they should not be created in the Castle, in fact NONE should be created there except in their respective military building (the trebuchet is a siege weapon, right?). It seems wrong to me to use the castles for this instead of putting them in their own military building creating an unnecessary and at the same time complex inconsistency.
All UUs share similar characteristics to those created in barracks, arches and stables so there would be no need to place them in castles.
I suggest that UUs be activated for use at the University with a generic option for all Civs for a cost and appear available in their corresponding buildings according to age.
IMPORTANT: Since yesterday some dudes flagged my comment, I will not respond to any of the comments to the present to prevent discussion since it seems that there is a censorship mafia that prevents free participation within the forums. It was my first comment and a very unpleasant experience that I hope will not be repeated.
Thanks and apologies.