I like to build forts at road intersections and choke points like passes and river crossings. In RTW I manned them with the cheapest and most expendable unit I could find, usually peasants. I haven't played M2TW much and had started to use the same tactic, until I noticed that peasants aren't really that much cheaper than real units, so I may try peasant archer garrisons.

I didn't have a problem with the forts in RTW (and especially BI) because they were very much in character with the spirit and practise of low Empire fortifications. And since the real limitanei were a pretty useless bunch, peasants as garrisons made sense and a good stand-in. I didn't even mind too much that the forts would vanish if not garrisoned, because that felt right. Besides, the way I was playing them the only time they would ever be empty would be the turn after an attacker moved out. So it made sense that the fort would vanish (i.e. no peasant maintenance cost for upkeep, no strategic flypaper). And it made the need to reconstruct the border defenses very tangible after a barbarian horde swept through.

Unfortunately the BI rationale for peasant garrisons in forts doesn't work in M2TW. Temporary field fortifications weren't as common in the middle ages as in Roman times. And the places that I build forts at would have been held by motte-and-bailey castles that would eventually convert into stone towers. [Actually, most of the places I build forts at would have been small towns, but let's not get too real]. If you travel around Europe today you can still see the ruins of these things, so they most definately shouldn't vanish overnight.

If I could wave my magic wand I would create three classes of fortifications: temporary camps, wooden towers, and stone towers. The temporary camp would be much cheaper but much more ephemeral that the present fort (when the army building it leaves, it goes poof). Wooden towers could probably work like current forts, but would have much better defenses and very limited garrison capabilty. Stone towers would be like wooden towers, but even more expensive and permanent.