I've got a tactic to add to your OP, involving Naval Warfare.
So you've got a nice big ocean surrounding your castle. Wonderful! But what if enemies launched a large naval assault, and were structuring cannons not too far away? Oh crap! Lets get some boats out there and stop them! Now you could just plop down some boats after jumping in the water, or you could have and underwater harbor! Using the underwater harbor (building it deep enough so enemies couldn't swim down into it) you could launch a large fleet within a split second, going full speed, and catching the enemy COMPLETELY unaware. Using this tactic, you could quickly and efficiently stop enemy naval threats.