let's make them more unique
Castles:
- more advanced types of equipment to buy (why the hell village can sell plate armor?)
- no fee for troops in garrison
- more space for garrison then in villages and towns
Villages:
- features should be linked to trade, something like much faster replenish rate of Supply and Demand (S&D) then in Castles.
Cities:
- should have all features of castles and villages but less advanced ones. E.g. less types of equipment then in Castles. Less S&D replenish rate then in villages, less space for garrison then in Castles, etc.
Your ideas?