Portland Castle is one of the chain of coastal artillery forts, known as Device Forts built by Henry VIII. It dates from between 1539 and 1540 and is of the earlier, predominantly circular, design.
Its purpose, in conjunction with Sandsfoot Castle on the mainland, was to provide sweeping cannon fire across Portland Harbour. Together they offered protection to ships moored in Portland Roads.
Unlike its cousin Hurst Castle, Portland is virtually unaltered from its Tudor state. The building of the Portland Breakwater meant that the Victorian and 20th century defensive structures were built further along the coast to the east.
The windows were enlarged when the castle was converted into a domestic residence by the Rev John Manning in 1816, and the roof of the Gun Room has been removed.
When English Heritage took over the property in 1955 they stripped out all the other later additions, to present the castle as it was when built, as far as possible.