Brownsea Castle was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII.
Traditionally the Crown had left coastal defences to the local lords and communities, only taking a small role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely.
Modest defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

In 1533, Henry then broke with Pope Paul III in order to annul the long-standing marriage to his wife, Catherine of Aragon and remarry.
This resulted in France and the Empire declaring an alliance against Henry in 1538, and the Pope encouraging the two countries to attack England.
Henry responded in 1539 by ordering, through an instruction called a "device", the construction of fortifications along the most vulnerable parts of the coast.
The immediate threat passed, but resurfaced in 1544, with France threatening an invasion across the Channel, backed by her allies in Scotland.
Henry therefore issued 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 44 feet (13 m) 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 height, but this was not achieved.
The blockhouse was surrounded on the seaward side by a hexagonal gun platform, with a moat around the other three sides and a 24-foot-long (7.3 m) drawbridge on the south-west side to allow access.

The construction was paid for by a combination of the Crown and the local town of Poole, who took on the responsibility of garrisoning and maintaining it; in the early years of Elizabeth's reign, the normal garrison was described as comprising six men and was equipped with eight artillery pieces.

Additional work on the gun platform costing £56 was carried out in 1548, and further work was carried out in 1552 to develop the castle's defences, at the cost to Poole of £133.
The castle needed regular investment: 101 piles were driven in during 1551, probably to combat coastal erosion, and in 1561 the town petitioned the Crown for help with further repairs and the provision of new cannons.
Another petition was made for similar help in 1571, resulting in repairs two years later costing £520 and requiring 4,000 tonnes (3,900 long tons; 4,400 short tons) of stone.
More work followed in 1585, including building an additional  4-foot-high (1.2 m) wall around the castle.

In 1576, Elizabeth I granted the castles of Brownsea and Corfe for life to Sir Christopher Hatton, making him the Admiral of Purbeck.
Hatton argued with the town of Poole, claiming that he had the right to search and inspect ships going into Poole Harbour, as well as the rights to the revenues from the local ferry service; he lost his legal case around the ferrying rights in 1581.
In 1589, the Bountiful Gift refused to pull in for inspection, arguing that it had had a valid pass to leave, and Brownsea Castle fired on the vessel, killing two of the crew.
The castle's captain, Walter Partridge, was tried and conflicted of manslaughter, but ultimately pardoned.
The rest of the island, but not the castle, was leased out to various landowners over the next few years.
Brownsea Castle was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII. Traditionally the Crown had left coastal defences to the local lords and communities, only taking a small role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely. Modest defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.
In 1533, Henry then broke with Pope Paul III in order to annul the long-standing marriage to his wife, Catherine of Aragon and remarry. This resulted in France and the Empire declaring an alliance against Henry in 1538, and the Pope encouraging the two countries to attack England. Henry responded in 1539 by ordering, through an instruction called a "device", the construction of fortifications along the most vulnerable parts of the coast. The immediate threat passed, but resurfaced in 1544, with France threatening an invasion across the Channel, backed by her allies in Scotland. Henry therefore issued 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 44 feet (13 m) 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 height, but this was not achieved. The blockhouse was surrounded on the seaward side by a hexagonal gun platform, with a moat around the other three sides and a 24-foot-long (7.3 m) drawbridge on the south-west side to allow access.
The construction was paid for by a combination of the Crown and the local town of Poole, who took on the responsibility of garrisoning and maintaining it; in the early years of Elizabeth's reign, the normal garrison was described as comprising six men and was equipped with eight artillery pieces.
Additional work on the gun platform costing £56 was carried out in 1548, and further work was carried out in 1552 to develop the castle's defences, at the cost to Poole of £133. The castle needed regular investment: 101 piles were driven in during 1551, probably to combat coastal erosion, and in 1561 the town petitioned the Crown for help with further repairs and the provision of new cannons. Another petition was made for similar help in 1571, resulting in repairs two years later costing £520 and requiring 4,000 tonnes (3,900 long tons; 4,400 short tons) of stone. More work followed in 1585, including building an additional  4-foot-high (1.2 m) wall around the castle.
In 1576, Elizabeth I granted the castles of Brownsea and Corfe for life to Sir Christopher Hatton, making him the Admiral of Purbeck. Hatton argued with the town of Poole, claiming that he had the right to search and inspect ships going into Poole Harbour, as well as the rights to the revenues from the local ferry service; he lost his legal case around the ferrying rights in 1581. In 1589, the Bountiful Gift refused to pull in for inspection, arguing that it had had a valid pass to leave, and Brownsea Castle fired on the vessel, killing two of the crew. The castle's captain, Walter Partridge, was tried and conflicted of manslaughter, but ultimately pardoned. The rest of the island, but not the castle, was leased out to various landowners over the next few years.
Brownsea Castle