Actually, I was just thinking about something!
I'm going to move the camp/fort (and the Roman Citadel) into a brand new category - "Fortress." This is different from "Towers." Fortresses are are expensive and slow to construct, but put down a very sturdy defense when they are done (and will do secondary things like store resources, train units. etc). "Towers" on the other hand are the opposite, they are much cheaper, easy to construct, and can be spammed relatively easily (also creating a sturdy defense). The "Defense Tower" would be a tower. Now, not every faction will have a Fortress defensive structure, but only the Njord will have no Towers.
Since they will have no "Towers," they're at a severe loss, since they can't defend a location without laying down a good deal of resources. I'm going to boost the stats of the Camp/Fort on the spreadsheet now, to reflect that they're going to be far above a Tower (in terms of defense), but just hard to acquire.