Maybe making the Bannor have "Bannor forts" which grow like a cottage offering greater and greater defense bonus as well as bonus commerce would be better than trying to turn the cottages into semi-forts. That way a useless improvement (fort) becomes useful. Double take.
Of course, that would make the Bannor have no need for Education, but... Having an educated population, well-learned and capable of questioning is probably not what they want anyway, huh?
Maybe something like...
Bannor Rampart (+1 commerce, +10% Defense). After worked ->
Bannor Fort (+2 commerce, +25% defense). After worked ->
Bannor Citadel (+3 commerce, +40% defense). After worked ->
Bannor Bastion (+4 commerce, +55% defense). After worked ->
Bannor Fortress (+5 commer, +70% defense)
Or something like that. If you spammed the ramparts and worked them like most people do cottages nowadays, the enemy probably would be forced to attack units in fortifications, and this fortification would really have a big effect, which rarely happens as is. Should make it so bombarding a bannor fort improvement would reduce it to one level lower (Rather than removing the improvement, like happens with a normal fort). That way, taking any "cottage" becomes a mini-siege. Serves the purpose of slowing down the enemy just as much, doesnt it?
I think drafting already serves the function of the militia that has been proposed.
Edit: The bonuses to defense should probably work in reverse if there is an enemy standing in the tile with the improvement. That represents the population of the cottage-fort fighting along with the soldiers, and makes taking a Bannor cottage even tougher - if you take it, you better raze it completely fast, or else reinforcements are going to come and take you out.