Hello
For What I know, Mordor players can build their 'fortresses' on the building pots for recruit his special units, but, if Mordor plays on the 'fortresses' maps of Dol Guldur and Minas Morgul, there doesn't have a building that recruit the units, but in Cirith Ungol map they can recruit his units in the building near the citadel. Why don't do the same. After all, it will be weird if in Minas Morgul map, you can only recruit the orcs of there building the fortress in a building pot OUTSIDE the walls. For me, that's has no sense for me.
For that I suggest to create a small building somewhere in the fortresses for recruit that units.