I'm thinking of having a flotilla of vulture-headed pygmies attack Nidus from several sides. The officers of the army will be orangutan lesser demons in howdahs. The party has no troops to speak of but since Nidus is pretty anarchic, if they take a leadership role I figure some people will follow them. That would include some red sash hirelings and maybe some foreign mercenary bands.
I'm going to try and play this out from what I gleaned from Zak's battle of the styrofoam cup bridge. There will be several battle hotspots and battle off screen will be handled in a fairly simplified dice off.
I would like the ares being attacked to be evocative and also have strategic consequences. Maybe an archway to the north of the city would allow the party to fight fewer of the beasties at a time. A broad stairway near the dock might allow them to save some of the ships being burned by the invaders. A small plain before a bridge into the city to the south would allow them to maneuver troops around if they really want to.
Of course they can also run off into the woods and leave Nidus to its fate, but then . . . no more shopping! hahah