In the case of the Coalhouse battery, it was substantially expanded between 1847 and 1855 to convert it into a fort. The work was slow as the marshy ground caused the foundations to crack and the structure to subside, and the contractor was unsatisfactory. The new fort was built as an extension to the north-west of the existing battery, and thus took an irregular plan. The rampart was extended to accommodate more guns – a total of seventeen 32-pdrs. – and a much enlarged interior replaced the old barracks and magazine. Caponiers at the east side and firing positions on the other sides facilitated musketry defence against land-based assailants. The fort was surrounded by a wide water-filled ditch. A bridge on the west (landward) side provided the only access route. By the late 1850s, Britain and France were locked in an arms race. A new generation of increasingly accurate and powerful guns had been developed (of the rifled muzzle loader (RML) and rifled breech loader (RBL) types), mounted on fast-moving, manoueuvrable steam-powered ironclad warships such as the French "La Gloire" and the British . Such vessels posed a serious threat to the important naval installations on the Thames, including the victualling yards at Deptford, the armaments works of Woolwich Arsenal, the shipbuilding yards at North Woolwich, and the magazines at Purfleet. It was not possible for large warships to reach central London, as the river was not yet deep enough to take ships of more than 400 tons above Deptford. (Capital dredging later enabled much larger ships to reach as far as the Pool of London.) As the American Civil War was soon to show, it was quite possible for the warships of the day to run past forts and attack up coastal rivers. The new weapons meant that the existing coastal and riverine forts were rendered largely obsolete. The American Civil War was soon to demonstrate that traditional brick and masonry forts could be reduced to rubble by rifled guns. The government's response was to appoint a Royal Commission on the Defence of the United Kingdom, which published a far-reaching report in 1860. The Royal Commission recommended that a triangle of forts should be established on the lower Thames, east of Gravesend. This would involve replacing the existing Coalhouse Fort on the Essex shore with a new fortification, similarly replacing the existing Shornemead Fort and building the wholly new Cliffe Fort opposite Coalhouse Point, which would replace the abandoned 18th-century battery at Lower Hope Point.