i can understand why building your own castle design has not been added. it is actually alot of work to make sure any random castle that a player might design would really work.

if you look at the stronghold number one game the whole engine was designed for you to place, block by block the whole wall. that made it easier to deal with complex curvature of the earth. the player must also be careful to place pieces right next to one another to assure, "no holes". this kind of thing can cause ctds unless the game to designed to work with such abnormalities that can occur. if you notice, most of the cities and castles in mtw as well as rtw are on completely flat ground. what happens to a long segment of wall that would be placed partially on a cliff face? that kind of thing has to be programmed.

i played the undone campaign in chivralic total war mod. it really wasn't completed and wasn't meant to be played. i attacked a city. once it was in the city map the whole castle was suspended 20 feet in the air. it crashed part way through the battle.

it would be a nice feature for some players, but it would take alot of manhours to make it workable.