Built in 1544, the Castle was part of a series of fortifications constructed by Henry VIII around England's coasts to protect the country from invaders. Barely was the work completed when Henry VIII's flagship, the Mary Rose, tragically sank in front of the Castle. During the English Civil War, nearly a century later, the Castle was captured for the only time in its history, by Parliamentarian forces.
Over the centuries, Southsea Castle's defences were strengthened so that it could continue to protect Portsmouth. In the 19th Century a tunnel was built to defend the Castle moat. Visitors can still enter the tunnel and see how the Castle would have been defended against invaders.
The Castle has had many other uses besides defence. For a while it was a military prison. A lighthouse was built in the 1820s, and is still in use by shipping today. In 1960 the Castle left military service. It was acquired by Portsmouth City Council, which restored the Castle to its 19th century appearance.