Overlooking the Orwell Estuary, the Fort has a long and rich history. Originally a fort was built at the behest of Henry VIII. The later fort of 1625 (made of turf with walls 30 feet thick) was the last fort in England to oppose a full scale invasion. It was on 2nd July (Julian Calendar) in 1667 when Captain Nathaniel Darell and 400 of the Duke of York & Albany's Maritime Regiment, with 100 gunners and 51 cannon, successfully repelled Admiral de Ruyter's Dutch force of 2 thousand! It is still remembered today.

Since then the Fort has seen many changes. The present brick Fort was started in 1719 and completed in 1720, but then completely redesigned and rebuilt in 1744.

During the 19th Century, Victorian engineers made massive improvements to the Fort, including the construction of a curved seven casemated battery facing the river. On two of the bastions, casemated batteries were also constructed and another on the SE curtain wall. Where there had once been a fifth bastion on the SW side of the Fort, a Caponier was built, with a large bomb proof shelter on the end.

The main armaments for the Fort were 12.5 and 10 inch Rifled Muzzle Loaded guns. Beneath the casemates and gun positions are the magazines, a labyrinth of passageways, shell, powder and cartridge stores. In some places the hoists for raising ammunition to the guns are still in situ. A separate passageway contained the lamps for lighting the magazines (keeping naked flames away from the potential hazards of powder and ammunition).

A large curved barrack and office section was built inside the Fort, creating an inner keep. High up in the corner between Harwich bastion and the curved casemated section can be seen the submarine minefield observation position with its thick steel roof. At the other end of the curved battery, again on the roof, is a concrete Fire Control tower. Built in the early 1900s, it had various sections added to it over the years. To the west of the Fort is Darell's Battery, originally built in 1901 and armed with two 4.7 inch guns. In WW2, two twin six pounder guns were installed. This area is at present fenced off for safety reasons.

South of the Fort is Right Battery, (built in 1901) it had two six inch and one 10 inch gun. During WW2, a further two six inch batteries were added. The whole of Right Battery is now within a bird sanctuary and can be visited by prior arrangement.

In collaboration with English Heritage, the Landguard Fort Trust has taken over the running and management of the Fort since the autumn of 1998. During 1997-98 extensive renovations were carried out on the structure of the Fort. The Trust run an ongoing programme to develop educational, interpretational and display materials within the Fort.