This is what the all-powerful Alex Walz said on this matter in the castle design suggestion thread about 10 months ago:
Beta testers will get to play around with the scenario design editor and who knows, maybe we'll take some fan-made strongholds, balance them up, and add them to the game.
Also, we will be revamping the wall construction. They'll still be preset, but you will have at least a dozen different formations you can build.
To explain... when a player is expanding his city, his gets to choose different selections on how the walls will be built, where the gates are, etc.. A player will get to choose from 12 or more wall layouts, as Alex says above. The reason it was setup this way is because the devs did not want any superfortresses that where impossible to take, which unfortunately, was one of the downsides of Strongholds.
As far as I know there are no "Castles" per se, only fortified towns and cities.
"Yes... I am writing a book. So far I have the page numbers." - unknown author.