A defended gateway - the Couvre Porte - had been built during the second phase of construction of the Chatham Lines in 1779-1786. During the third main phase of construction (1803-1815) during the Napoleonic Wars, the gateway was converted into a hornwork with guns to provide enfilade fire along the lines. Buried gun chambers were also installed under the rampart to fire along the ditches of the hornwork. The southern half of the hornwork was remodelled in the late 19th century and during the Second World War was converted into an emergency reservoir.