The thing is, there are so many more options when it comes to castles. Here are just a few, ranging from the vaguely real to the completely made up.
A few real options:
- A traditional motte and bailey arrangement, with two separate bits, one of them on a hill.
- A concentric set up with multiple rings of walls. Though for fantasy, this might be a bit like a neat bullseye for a passing dragon to hit.
- Something consisting of just a keep, the way many Scottish castles do.
- Something with multiple interlocking courtyards, like cells of a beehive, so that you have to invade them one at a time.
- A country house with castle-y bits on, for the look of the thing.
- A castle made of some odd substance (I'm currently thinking sand)
- A castle that is actually the abandoned shell of a giant hermit crab
- A castle with no apparent walls or windows, that you can only walk through the walls of with the right key.
- A 'castle' made of fragments of lots of other people's, spread throughout time and space.
- A castle that started to get National Trust style renovations, but had so many that only the scaffolding remains in a vague castle shape.