About castles:
Tyrant has some interesting ideas, but I for one don't want to see a huge amount of new building options (micromanagement), so how about this: castles provide defensive structures to the adjacent provinces, earthworks, outpost and such (will appear in battles when in such province) AND improve PD to "class 2", castled provinces will have better PD (crude example: class 1 -> militia, bowmen, class 2 -> heavy inf, longbowmen). An Outpost would have zero extra defensive structures in adjacent provinces and no PD upgrade, Wizard's tower would have zero bonus defensive structures in adjacent provinces but upgraded PD and a Castle would have the structures and the PD. Add the existing differences in castle types and the choice of a castle becomes much more important. Just an idea, feel free to comment, add to and improve, all in the name of the game we all love