Dover Castle is situated at Dover, Kent and has been described as the "Key to England" due to its defensive significance throughout history. Massive rebuilding took place at the end of the eighteenth century during the Napoleonic Wars. William Twiss, the Commanding Engineer of the Southern District, as part of his brief to improve the town's defences, completed the remodelling of the outer defences of Dover Castle adding the huge Horseshoe, Hudson's, East Arrow and East Demi-Bastions to provide extra gun positions on the eastern side, and constructing the Constable's Bastion for additional protection on the west. Twiss further strengthened the Spur at the northern end of the castle, adding a redan, or raised gun platform. By taking the roof off the keep and replacing it with massive brick vaults he was able to mount heavy artillery on the top. Twiss also constructed Canon's Gateway to link the defences of the castle with those of the town. With Dover becoming a garrison town, there was a need for barracks and storerooms for the additional troops and their equipment. The solution adopted by Twiss and the Royal Engineers was to create a complex of barracks tunnels about 15 metres below the cliff top and the first troops were accommodated in 1803. At the height of the Napoleonic Wars, the tunnels housed more than 2000 men and to date are the only underground barracks ever built in Britain. At the end of the Napoleonic Wars, the tunnels were partly converted and used by the Coast Blockade Service to combat smuggling. This was a short term endeavour though and in 1826 the headquarters were moved closer to shore. The tunnels then remained abandoned for more than a century.