Portland Castle was built between 1539 and 1540, along with Sandsfoot Castle on the cliffs opposite, to protect an important anchorage known as the Portland Roads. It was one of a chain of artillery forts built for Henry VIII to protect the south coast from the threat of invasion by the French and Spanish following Henry's break from the Catholic church.

The castle is shaped like a segment of a circle, with a two storey circular tower at its centre and rectangular wings radiating from either side. The main gun batteries were on two levels in the curved front towards the sea, and a third tier of guns could be mounted on top of the central tower and its wings. The main gun room is now open to the sky having lost its roof that acted as the second gun platform.

The castle went through periods of neglect and repair, with its garrison and firepower bolstered whenever invasion was threatened. In the early 19th century the castle was converted into a private house by the Manning family. By the end of the century the castle was back in military hands and served as an army residence. The castle remained in use until after the Second World War.