The castles are a hybrid tab system / system-x, but the weak system-x.
There's the versatile system-x, with lots of useable parts like the school, hotel, church, hospital. And there is the weak system-x: castles with large prefab parts with lots of irregular forms. You can't do much with it: no roof extensions, no wall piling, no seperate battlements, etc. The towers are not system-x at all, except for the connectors to the walls. The square towers are made of a steck-like tab system. The round tower of the large castle is made of two clip system parts.