Its nucleus is one of the coastal forts of Henry VIII, expanded as a result of another invasion scare in Victorian times. The original castle was built in 1539-44 and the master mason, Thomas Bertie, later became captain of the garrison here, a curious but not uncommon reward for a castle builder.
Like Calshot, it lies at the end of a spit of shingle, well over a mile long and projecting into the middle of the Solent. The Isle of Wight is little more than a mile away and, along with its counterpart at Yarmouth, the castle's guns could effectively command the western entrance to the Solent.
Hurst was garrisoned almost continuously until the Second World War. Its situation also made a secure prison, used mainly for the incarceration of Catholics though its most famous inmate was Charles I en route to his trial and execution. The Henrician fort is now flanked by two long batteries added in 1861-73, when the fear of a resurgent France under Napoleon III led to that vast array of defensive works known as "Palmerston's Follies'.
Henry's castle is made up of a central tower, polygonal outside but circular within, surrounded by a thick curtain with three semi-circular projecting bastions. Large gun ports in the beginning pierced the curtain and further cannon could have been mounted on the parapets of the curtain and the higher central tower. Later modifications have obscured much of the original layout.
The central tower has a spiral stair turret at its nucleus, probably an original feature though it was rebuilt in the Napoleonic period when the tower's brick vault was inserted. Only the northwest bastion, which is higher than the others, preserves its original appearance. Beside it is the entrance gateway, retaining its portcullis groove and slots for the drawbridge chains.