A thought on moats, perhaps in the siege menu you could get an option to attempt to fill in parts of the moat, this would obviously cost time, money, and men, but would make the walls accessible at a few key points.

It would be awesome to be able to custom build castles. What would be cooler would be if you could choose whereabouts to place them on the campaign map as well... though that would probably unbalance the game in terms of the AI being utterly useless at placing them. UNLESS you went back to all settlements being cities, and implimented some rule whereby you could only have one castle per 'x' cities, to prevent people making their own personal margiot line...