Brownsea Island by third parties, is owned by the National Trust. Most of the buildings are situated near the small landing stage. The northern portion of the island is a Nature Reserve managed by Dorset Wildlife Trust and an important habitat for birds; this part of the island has limited public access. A small portion to the southeast of the island, along with Brownsea Castle, is leased to the John Lewis Partnership for use as a holiday hotel for staff, and is not open to the public. The island forms part of the Studland civil parish in the Purbeck local government district.
Brownsea Castle another device in 1544 to further improve the country's defences, particularly along the south coast. The castle was built on the south-east corner of Brownsea Island between 1545 and 1547 to protect the entrance of the busy Poole Harbour. The island belonged to the Crown, having been confiscated from Cerne Abbey during the Dissolution of the Monasteries a few years before. It was a conservative design, being a one-storey, square blockhouse, reported in 1552 to be across, able to support guns on its roof and sub-divided into three rooms; the blockhouse was originally intended to have been two storeys in