In vanilla MTW the AI never assaulted castles without seige weapons, i don't think it changed with the 1.1 patch. In VI & patched VI the AI will always try to assault as soon as possible. Sometimes they might wait a year but they do assault. I have fought a lot of seige defences & the AI is useless at assaulting anything but forts. I have defended geogia with a fortress & 1 lone JHI unit for 4 years. The horde lost i'd say about 4000-5000 troops tryin to take it over the 4 years.