The rise of Napoleon III during the 1850s caused a further invasion scare, and a Royal Commission was set up in 1859 to investigate the defences of Britain. As a result, more work was deemed necessary at the Heights, and the Drop Redoubt had its defences improved. Caponiers were added to four of the corners of the existing fort (each with a stone staircase leading up to the top of the Redoubt), and gunrooms were built alongside two of them to allow fire along the North and South-East Lines. The original magazine was enlarged, and covered with a large earth bank as protection from mortar-fire.