The Middle Ground Coastal Battery was once part of a reef, like the nearby Oyster Rock. Middle Ground islet is a piece of hard basalt rock, emerging from the creek bed, having an area of a few hundred square metres. The RIN added to the fortifications with the use of cannons and later on three anti-aircraft guns to protect the harbour from a perceived Japanese invasion during World War II. The AA guns were eventually removed, but the deep cylindrical mountings of the old guns still remain.