Personally, I would rather have been able to build fortifications at towns and ports rather than building regular forts. Especially considering that those fortifications express as fort battles (as opposed to the "walled city") and you can only build forts in friendly territory.

FYI, M2TW has regular woodens forts like RTW that disappeared after being unoccupied for a year. Kingdoms introduced permanent forts (as small castles). Modders added these to the grand campaign with Kingdoms based mods. It took a lot of tuning to make sure the AI didn't spam them everywhere apparantly...