I have been searching info on the same subject without much success, for what i understand, the models for castles are .3xx files, though i am yet to encounter a 3d modelling program that makes 3xx files. other files are also used but seem to be easily edited with a notepad or equivalent, im not sure if you have come across the this thread which describes how to modify the texture of the existing castles
The Guild > Medieval Totalwar > MTW modification > Repository> Modding Buildings, Walls and Gates
it may be of use to you...
if you find a way to bring in new models into the game, do pls post it as the work im currently developing requires a complete new revamp of the castle shapes and other objects such trees and houses

tx