Here's a suggestion (as if we need any more
) for how to make castle-types more flexible, while retaining their importance re. design points:
How about if the castle you pick when designing your pretender signifies something like the 'maximum tech level' achievable by your culture? In other words, when you build a castle you always first take 2 turns to build a watchtower, or perhaps mausoleum, for 300 gold. Then subsequent to that you can choose to 'upgrade' your basic structure to the castle you picked in god-creation, for however much time and gold.
You'd still retain the advantages of choosing an expensive castle-type, because you could build to it whereas others couldn't, but you'd have an interim step, and the cost broken up into a couple of chunks.
I'm not suggesting being able to build multiple buildings - I think that would mess the game up. Instead trying to think of ways to make castles like the MC more viable.