 Two spigot mortars were installed at the fort's rear; their bases can still be seen today.A detached concrete observation tower was built just north of the fort for use as an Extended Defence Officer's Post, to control the electrically detonated mines that had been laid in the river.A radar tower manned by naval personnel was also built south of the fort by 1941 to cover the approaches to the minefield, and an anti-aircraft battery was built to the north-west at Bowaters Farm in August 1939.