[HoneyBadger]

While we're on the subject of castles, how about the ability of natural fortification?

Natural fortification works like this:
In a forest, a cave, a mountain, a kelp bed, a coral reef, any place that happens to offer the right conditions for a defender to have a distinct advantage over an attacker (and not every forest, cave, etc. qualifies) you can "hide" troops behind such a thing, like a castle, but without the Defense factor.

So, in one province with Natural Fortification, you can have 1 set of units "patrolling", and one set of units "defending".

The "patrollers" combine with the PD and are attacked first, while the "defenders" (researchers, blood-hunters, what have you) remain safe behind the trees, stalactites, high ground, whatever it is that makes that particular province special.

Attackers can still seige the place after they've beaten the patrollers+PD, and can immediately attack the Defenders, but can't hurt defending units before that happens.

Defenders don't get a defense bonus, a supply bonus, administration, or any of the benefits of a castle, except the ability to split their forces and delay an enemy an extra turn.

Now, if you DO build a castle on that particular province, you get a bonus to the castle's defense of 25% to reflect taking advantage of the natural suitability of that particular province for defense.