Originally posted by esbenmf:

Dig a moat.

I should have been more precise in my description. I'm completely surrounded by a moat on all 4 sides, 8 m deep. However he was clever enough to find the narrow causeway. Without that causeway, I can't access the base myself !!! As I said, a pair of wooden doors usually discourage the lower level mobs. If they tear one down, the other invariably stops them.