From Wikipedia: http://en.wikipedia.org/wiki/Galle_fort

Galle is located at the extreme southwest corner of the island, where the shoreline turns east towards Matara and Tangalle. The Fort, like most of the forts in Sri Lanka, is built on a small peninsula, belonging to the sea as much as to the land. There are treacherous rocks in the water near the fort, and a treacherous current, so a pilot was needed to approach it. The seafloor here is littered with shipwrecks. The only way to attempt a conquest was to attack it from the landside, where the Zon, Maan and Ster bastions are impenetrable.