I saw one other post about this, however, no reply.
When I am placing my castle blocks, sometimes the walls shift down a tick after I leave and return which leaves a space between the top of the wall and the roof. I have placed all of the walls consistently so I'm not sure what I'm doing wrong.
To resolve this issue you'll need to pick the castle block up and set it back in place where you want it without moving it around. The problem is caused by setting a piece where it should be "interior" and then moving it to a place where it should be "exterior" and vice versa. If it was originally set in an "exterior" position and then moved to "interior" it will be "interior" until you leave and come back, then it will switch to "exterior." This is why some pieces grow larger and some pieces shrink.