My Historic Castles Project has been converted to multiplayer by Beaver at Taleworlds,
and is now hosted by IG (Guard of Istiniar) clan, on the server: IG_SiegeServer.
(Use native and the castle maps will automatically be downloaded when joining the server)
There are 9 castles. Here are screenshots:
http://www.twcenter.net/forums/showthread.php?t=359062
Some of them are actually quite fun