After testing, I've found a few bugs with the slots which lead to a game crash when trying to recruit a hero. I hope it's now corrected.
I have also modified a bit the building. In any region in Ulthuan you can have barracks and stable up to level 4, and can recruit elite generic units (like Sentinel spearmen).
Out of Ulthuan, you are limited to level 3 barracks/stable, and can recruit elite units.