I would say there is good reason to believe there are garrison-able buildings. Let me make the case
-Firstly it has never truly been removed since Empire: In Empire/Napoleon you could garrison farm buildings, fortifications and drystone walls, and in Shogun 2 you could garrison gatehouses and parapets. Thus we have no reason not to believe garrisoning would not be possible
-Secondly this was an age of city sieges and fort conflict. Fighting from walltops and in buildings, and designing fortresses was so common it would not make sense for all fights to be out in the open
-Thirdly, and I think most decisively, screenshots from the battle of Carthage showed troops fighting from houses as well as the normal wall and street battles, and they stated (though early enough in the development that I grant it could have been removed) that it would be in:

(bottom left of second pic)
See that the troops in the second picture are
inside, not on top of, the tower