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 HMS Warrior.
 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.
The locations of the forts would enable interlocking arcs of fire from their guns.
A boom defence and a minefield would be installed off Coalhouse Point in wartime to further boost the strength of the defences.

The design of the new fort was similar to that of the other Royal Commission forts on the Thames, with an arc of granite-faced casemates, reinforced by iron shields.
These, it was believed, would be virtually invulnerable to enemy fire.
It was originally envisaged that the fort would have two tiers (in a design similar to that of Garrison Point Fort at Sheerness) mounting around 56 guns; 28 in casemates and the rest in barbettes on the fort's roof.
Construction began on this basis in July 1861 but as the work progressed the design was changed, leaving the fort with only a single tier of casemates.
Like its predecessors, its construction was seriously affected by the poor ground conditions and was disrupted by shifting and cracking foundations.
Colonel Charles George Gordon, who was later to die in the Siege of Khartoum, supervised its final phases of construction.
The fort cost a total of £130,000 to build.
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 HMS Warrior.  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. The locations of the forts would enable interlocking arcs of fire from their guns. A boom defence and a minefield would be installed off Coalhouse Point in wartime to further boost the strength of the defences.
The design of the new fort was similar to that of the other Royal Commission forts on the Thames, with an arc of granite-faced casemates, reinforced by iron shields. These, it was believed, would be virtually invulnerable to enemy fire. It was originally envisaged that the fort would have two tiers (in a design similar to that of Garrison Point Fort at Sheerness) mounting around 56 guns; 28 in casemates and the rest in barbettes on the fort's roof. Construction began on this basis in July 1861 but as the work progressed the design was changed, leaving the fort with only a single tier of casemates. Like its predecessors, its construction was seriously affected by the poor ground conditions and was disrupted by shifting and cracking foundations. Colonel Charles George Gordon, who was later to die in the Siege of Khartoum, supervised its final phases of construction. The fort cost a total of £130,000 to build.
Coalhouse Fort