Build a moat. Just wait until you defeat the Elder and you start getting nightly visits from tag-teams of trolls. They can tear thru stone walls like butter. The only thing that defeats them is an 6-8m high earthen wall, and even then they can reach over the top and smash stuff within 8m.

Boats have physics....... things in motion tend to stay in motion. Boats will not stop on a dime like a car with good tires.