i have run one siege of a town. essentially each round was a attack from the invaders (rats in my case).
so for the defenders, an attack would be essentially be a counterattack, archers, or a stoic defence of the ramparts. feinting is easy, its a bluff of some kind so letting the enemy in, closing the gates and surrounding them. maneouvering is clever placement of troops, or sending out skirmishers to cut off supply lines. (little tip - look at the weapons used for wars. how could they be disrupted?) defence is a tricky one, because if youre defending against a siege youre techinically always on the defense. you could simply call this ‘weathering the storm’, or reinforcing with your reserves.
for my siege in particular, the scale was quite small. (think rorkes drift small) and so i allowed the players to defend with healer, and attack with fighter (maybe at -2D, i forget now) to represent leading from the front. on a grander scale, even a great hero can make a limited difference by martial prowess alone. i may also have allowed orator for an action if appropriate.
i did all this so that the players without militarist/administrator still had stuff they could do, but at an appropriately limited impact.
they key here is making sure the players know what their actions represent. hopefully then they will pick actions/descriptions that fit in, and dont spoil the flow of the battle.
have each action have a tangible result - defending a wall? failure results in the loss of the wall. use your next action to retake it. this way persistent failure will result in the players having a real feeling of what their disposition means. for the attackers it is essentially how many more troops you can throw into the battle. this way you can see a defence as the evacuation of civilians, the construction of barricades, and so on.
make sure players narrate their weapons. if they want their +!S to defend, they have to tell you how they are getting it. the mechanics should inspire narrative, not merely a number.
i can also recommend looking up some battles on wikipedia, which is pretty good for an overview. you can see there the sort of interplay between generals, and imagine what sort of action (AFDM) they might be. (example, charge of the light brigade - feint vs attack)
lastly, basic prep stuff. have a map of walnutpeck. have an idea of how large the forces involved are, and the quality of the defences. make sure the weasels have plans and strategies to make them seem like a real enemy. if the mice are doomed to fail, make sure their conflict goal is something fatally dramatic, like ‘escape with as many mice as possible’, or hold back the attack long enough to allow an escape’, or possibly just ‘survive til dawn’ (night fights with fog are the scariest). even if overall failure is inevitable, give the players a shot at glory, perhaps a final cavalry charge into the weasel ranks to take on the Overlord 1 on 1?