I've played campaigns on all settings (most on VH/Medium) and the AI will attack forts on whatever setting, it does seem very reluctant to do so though. Doesn't really seem to matter how small the garrison is if theres a fort AI generals will 90% of the time walk up to it and then leave, 5% of the time they will beseige it and then lift the seige within a couple of turns never to return. The other 5% of the time they actually try and take the fort. IMO.