Gruumsh views on keeps and cities and the like:
This is just the guess of an LOTR freak, but I think that orcs and goblins and other critters like that can also breed in mountain ranges/caves, etc. (hence the route via mountain range in The Hobbit and the Mines of Moria in FOTR). Such underground places have massive capacity, so you can have all the city massing hordes you want

. However, not sure if this is really the case for orcs. Certainly for goblins, but Tolkein often uses the words interchangeably. Alright, in terms of FR language, I
think they can breed in those large unmapped mountain areas out there, especially because they often seem to come in from the north and all.