The Island was used as the Union’s coastal defence as evidenced by the high concentration of WW2 sites. Construction of the first of the Island’s two main gun emplacements was completed in April 1940. This was a 6” calibre gun in the Cornelia Battery which covered the Blouberg Channel. The second coastal battery, known as De Waal Battery (previously called Robben Island Battery) was added in 1942. Here 9.2” calibre guns were installed in November 1940 and July 1941.
Other construction on numerous sites around the Island included plotting rooms, an engine room, observation posts, fire control towers, search light sites, storerooms and underground bunkers that stored ammunition. A network of tunnels connected an extensive system of underground bunkers including a hospital, a power plant and ammunition dumps. Construction of the Murray’s Bay harbour was completed and some new roads were built. The airstrip was established and cables linking many new structures were laid. Pillboxes and rifle posts were added at the same time that the airstrip was being built.