The 38-ton RML (Rifle Muzzle-Loading) 12.5-inch gun was used by the Victorians for coastal defence. 

Landguard Fort occupies a strategic position overlooking the navigable channel into Harwich Haven. It stands on the site of the last opposed seaborne invasion of England, by the Dutch in 1667. The invaders were defeated by the Royal Marines in their first land battle. The current fort was built in 1744 to a pentagon plan with a bastion in each corner.

It was extensively modified throughout the 1800s, which continued into the following century as the fort's armament grew obsolete. It was manned throughout both World Wars, latterly as an anti-aircraft role during the WW2. In 1951 two gun casemates were converted into a Cold War control room. The fort was abandoned by the military in 1956. Today, it is open to the public and run by the Landguard Fort Trust under the stewardship of English Heritage.