[NTJedi]

No matter how strategic of a great location for a castle... some territories should be prevented from building castles for other reasons. One this will allow map makers to be more creative with designing maps thus having provinces exist where no castles can be built... otherwise map makers won't have this option. Two it's realistic that certain terrain exists where no castle will remain standing from the effects of the land and mother nature. Three this will provide an additional obstacle within the game where players will have to be more creative in defending these provinces.

I agree about improving province defense as well... here's a copy of my ideas posted from the Dom3 wishlist:
Improving Province Defense

One or more of these ideas can be used for improving province defense:

A) Provinces adjacent to the main capital and including the main capital have very powerful defenders for its province defense. These units should have much higher morale, defense, magic resist, and protection.

B) More unique and stronger defenders as provinces defense increases. Province defense(1_thru_10) basic units, Province defense(11_thru_20)average units added with basic units, Province defense(21_thru_30)strong units added with basic and average units, and so on...

C) For province defense beyond 20... adding a commander with a standard(+8) and additional commanders with standards for every additional 10pts of province defense.

D) Some unique powerful unit(depending on the race) added for any location with province defense of 50 or more.

E) Increase resources in a province when the province defense goes beyond 25, yet doesn't work on capital. This would give province defense a second value. Province defense beyond 25 is an investment of over 350 gold.