To stop IMMEDIATE attacks couldn't you edit the one of the walls to be able to be torn down and have some stupidly low cost. Enemy sieges, give yourself a a few turns tops to reach the city to send reinforcements, if you can't make it there in time, destroy the wall. It becomes a bit more involved, but at least you won't have to have major amounts of troops spread across the entire map. Though if they attack before you can destroy them, your at the same problem as before.

Still playing with walls currently - but I'm starting to debate this at times. The AI in 1.2 is too conservative in attacking fortifications more often than not. Sometimes it'll manage to stumble itself into attacking and attacking decently - but not often.