It was originally intended that Coalhouse Fort would be armed with 68-pdr.
smooth-bore guns, but these had become obsolete by the time it was completed in 1874.
More powerful armour-piercing weapons were required, so the casemates were provided with four 12.5-inch and thirteen 11-inch RMLs with a range of about 5,500 yards (5,000 m).
A further three 9-inch RMLs were emplaced in the open battery.
They were mounted on metal traversing platforms that could be elevated and traversed using hand-operated gearing devices.
Operating the guns presented significant challenges; the 12.5-inch RMLs weighed up to 38 long tons (39 t) apiece, used a charge weighing 172 pounds (78 kg) to fire a shell weighing 827 pounds (375 kg) to their maximum range, and broke windows half a mile away when they were fired.
The detonations and clouds of choking black smoke generated when firing them presented a serious physical hazard to the gun crews.

The guns were controlled from a Battery Commander's Post on the roof, from where commands to the gun detachments could be relayed via voice pipes.
Three depression range finders were installed on the roof by the late 1890s to aid the guns' targeting.
A well-trained crew could fire each gun once every two minutes.
Rather than firing all the guns at once, the guns were positioned so that they could be fired in sequence as an enemy ship passed by.
When combined with the fire from Shornemead and Cliffe Forts on the Kent shore, this would ensure that constant fire could be maintained from three different sides.

The rapid development of artillery weapons in the late 19th century meant that the casemate style of fortification had become virtually obsolete within a decade of the fort's completion.
Its original purpose of defending the river against large warships was taken over by a new East Tilbury Battery, constructed in 1891 about 600 metres (660 yd) to the north-west just outside East Tilbury village.
This contained six much more powerful breech-loading (BL) guns on Moncrieff disappearing carriages within concealed emplacements facing east up the river.
 A similar battery was built at Slough Fort on the Hoo Peninsula a few miles downriver.

At the same time, a new threat had emerged in the shape of the fast and highly manoeuvrable torpedo boat.
Large guns were virtually useless against such vessels, so the fort was supplemented with another new battery built in 1893 some 365 metres (399 yd) to the south where 6-pdr.
quick-firing guns were installed.
Similar batteries were built on the opposite shore at Shornemead and Cliffe Forts.
It was originally intended that Coalhouse Fort would be armed with 68-pdr. smooth-bore guns, but these had become obsolete by the time it was completed in 1874. More powerful armour-piercing weapons were required, so the casemates were provided with four 12.5-inch and thirteen 11-inch RMLs with a range of about 5,500 yards (5,000 m). A further three 9-inch RMLs were emplaced in the open battery. They were mounted on metal traversing platforms that could be elevated and traversed using hand-operated gearing devices. Operating the guns presented significant challenges; the 12.5-inch RMLs weighed up to 38 long tons (39 t) apiece, used a charge weighing 172 pounds (78 kg) to fire a shell weighing 827 pounds (375 kg) to their maximum range, and broke windows half a mile away when they were fired. The detonations and clouds of choking black smoke generated when firing them presented a serious physical hazard to the gun crews.
The guns were controlled from a Battery Commander's Post on the roof, from where commands to the gun detachments could be relayed via voice pipes. Three depression range finders were installed on the roof by the late 1890s to aid the guns' targeting. A well-trained crew could fire each gun once every two minutes. Rather than firing all the guns at once, the guns were positioned so that they could be fired in sequence as an enemy ship passed by. When combined with the fire from Shornemead and Cliffe Forts on the Kent shore, this would ensure that constant fire could be maintained from three different sides.
The rapid development of artillery weapons in the late 19th century meant that the casemate style of fortification had become virtually obsolete within a decade of the fort's completion. Its original purpose of defending the river against large warships was taken over by a new East Tilbury Battery, constructed in 1891 about 600 metres (660 yd) to the north-west just outside East Tilbury village. This contained six much more powerful breech-loading (BL) guns on Moncrieff disappearing carriages within concealed emplacements facing east up the river.  A similar battery was built at Slough Fort on the Hoo Peninsula a few miles downriver.
At the same time, a new threat had emerged in the shape of the fast and highly manoeuvrable torpedo boat. Large guns were virtually useless against such vessels, so the fort was supplemented with another new battery built in 1893 some 365 metres (399 yd) to the south where 6-pdr. quick-firing guns were installed. Similar batteries were built on the opposite shore at Shornemead and Cliffe Forts.
Coalhouse Fort