The castle, built on a shingle spit near Rye, East Sussex, to defend the river Rother, has been restored by English Heritage, its crumbling walls made safe with lime mortar. Strange underground passages beneath its courtyard, where soldiers once prepared to repulse the Spanish Armada, may now be explored by children - or by adults if they are prepared to bend double.
Camber Castle is today a mile or more from the sea and cows graze in the meadows around its walls. The castle was built in 1539 as part of a chain of coastal defences to prevent the Catholic powers of Europe from invading England.
It was abandoned in 1637 because the river altered course and its guns no longer commanded the harbour mouth. So Camber Castle has remained unaltered, whereas other castles in Henry's chain of coastal defences, Calshot in the Solent, Walmer, Deal, Sandown, Dover and Sandgate, continued to be used into the 19th century and so were altered.
Recent excavations have found that the castle was built according to a revolutionary design by Stephen von Haschenperg. Gun forts of its type were new to England at the time.
Jonathon Coad, inspector of ancient monuments for English Heritage, said: 'We found from ancient records that Camber Castle cost three times as much to build as other similar castles and when we excavated it, we found out why. It seems that von Haschenperg used a novel design which was an amalgam of two different types of gun bastions used on the Continent. The design did not work and so the bastions had to be rebuilt at great expense.'
The bastions at Camber were built with large vents to take away the smoke made by the exploding gunpowder. 'The fog of war was very real in those days. Smokeless gunpowder did not come until much later,' Mr Coad said.
Soldiers had access to the bastions by means of underground passages, which also provided a second line of defence should the outer wall be overrun. A series of slits in the tunnels enabled the defenders to fire at invaders scaling the wall or penetrating the courtyard. Coastal forts built by Henry VIII a few years later do not have these tunnels.
'It must have been decided that the main function of these forts was the defence provided by their large guns,' Mr Coad said. 'Camber was experimental because it was the first.'
As the sea receded, the river mouth was no longer in range of the Camber cannon. At the same time, tension increased with Spain and with it the danger of an invasion. So the guns were raised about 15ft by filling in the bastions with shingle.
But by 1637 the guns were once more out of range. The garrison was withdrawn and the lead stripped from the roof.