Greater Sober Island is the large of two islands that lie across the entrance to the natural harbour at Tricomlee. Because of their position they were used for coastal defence batteries by the British. It would appear that the Dutch had relied on Fort Ostenburgh on the mainland.
The North end of Greater Sober Island was used for residential accommodation for senior naval officers. There is a clear demarcation line with Boundary Stones, between the Admiralty (Royal Navy) and Army sections of the Island. A pier was built on the North end of the Island allowing for a speedy boat trip to the main naval base below Fort Ostenburgh.
The South end of the Island has three coastal defence batteries and these were :
Arthur Battery, initially three 9-inch RML and then one 9.2-inch and later two 6-inch BL guns
Wellesley Battery with four 40-pdr RBL guns
Green Battery with two 6-pdr Q.F. guns