Southsea Castle was one in a series of forts constructed for King Henry VIII, in what was the most ambitious scheme of coastal defence since Roman times.

The castle was built in great haste in 1544, prompted by Henry VIII's fears of a French attack on Portsmouth. It was said to have been designed by the King himself, incorporating the latest continental ideas on the lay-out of artillery forts. It was not long finished when on 18 July 1545, a French invasion fleet did approach Portsmouth and landed on the Isle of White. Henry VIII was at Southsea Castle when the Mary Rose sank next day.

The castle was to be an active military base for over 400 years. Life there had its ups and downs. In 1627, for example, the keep was gutted by fire. Then in 1642, at the outset of the English Civil War, the Royalist commander was almost too drunk to surrender when a large Parliamentary force surrounded the castle. Fortunately he sobered up enough to do so, and there was no loss of life.

Seventeen men, women and children died, however, in 1759 when an accidental explosion blew up a large part of the castle. It became so dilapidated that it was nearly demolished, but in 1814 it was completely renovated to accommodate extra guns and a larger garrison in time of war.

The castle's key position guarding the entrance to Portsmouth harbour meant that whenever danger threatened it was right in the front line. Even when it was used as a military prison during Victorian times, its guns still had to be ready for action. Not until 1960 was the castle finally withdrawn from active service and purchased by Portsmouth City Council.