18 x 18 houses are already customisable, Castles and Keeps are not, so that's why they have a contest to make other designs available. The classic castle and keep are just not very practical or easy to decorate, yes some of us would have preferred a re-design of the classic interiors to fix the problems, but the Devs chose to go a different path. A re-design would have taken a lot less Dev time than going around even one set of contest buildings, my husband already sent them floor plans for the castle and keep to be fixed with very little work, I know other people did too, but they were not interested in doing it themselves.
If you just want ideas for building an 18 x 18 there are quite a few on the forums.