The island was fortified by Henry VIII, and the castle he built became a parliamentary stronghold in the English civil war. The castle was partially demolished after a fire at the end of the 19th century.
The island is one of the few places in south England where red squirrels survive, as grey quirrels have never been introduced to the island. The island contains a variety of habitats, including a saline lagoon, saltmarsh, reedbed, two freshwater lakes, alder carr, pine woodland, deciduous woodland and arboretum, and is designated a Site of Special Scientific Interest.
In the Second World War lights were placed on the island at night to divert the attention of bombers away from the nearby conurbation of Poole and Bournemouth, army camps on the Purbecks and the important naval base on the Isle of Portland. There are many bomb craters on the island which are havens for rare wildlife.