Southsea Castle was one of a series of artillery forts built by Henry VIII along the south coast of England to guard against invasion from Europe. Unlike earlier round bastion designs it incorporated angular defensive walls. Standing at the Eastern Solent at the head of the deep water entrance into Portsmouth Harbour, it is famous for being the viewpoint from which the King witnessed the foundering of his flagship, the Mary Rose in 1545.

The castle remained in military use for more than 400 years. It fell to  Parliamentarian soldiers in 1642 during the English Civil War but now bears the coat of arms of Charles II over the entrance. The lighthouse was added in 1828. The castle was withdrawn from active service in 1960 and purchased by Portsmouth City Council.