The castle was garrisoned throughout most of the 17th century.
In the civil war of the 1640s between the supporters of Charles I and Parliament, the castle was held for Parliament, under the control of the Governor of Poole.
The castle was refortified and in 1644, Parliament ordered that four pieces of artillery and four chests of muskets be sent to the castle, which by 1646 had a garrison of 20 men.
During the Interregnum, the wealthy merchant Sir Robert Clayton bought the surrounding island.
Clayton probably did not live in the castle, however, and it fell into disrepair; by the end of the century the town of Poole refused to garrison the decaying defences.

The amateur architect William Benson bought the island from Clayton's heirs in 1726 for £300.
Benson set about converting the castle into a private residence, amid complaints from the authorities in Poole.
The town took the matter to the Attorney General, where they argued that Benson had not bought the rights to the castle itself, only the island, and that the castle was a national fortification, originally built by Henry VIII and owned by Poole.
Benson argued that the building had not originally been a castle but rather a lodging house; it had not been built by Henry and therefore the Crown had no particular rights over it - rather, the previous owners of the island had simply allowed the town and the government to place artillery there.
The matter was eventually dropped and Benson demolished the external fortifications, created a Great Hall and planted trees and rare plants around the island.

The castle was sold to a Mr Chamberlayne and then onto Sir Gerard Sturt in 1762 and Gerard's cousin, Sir Humphrey Sturt, in 1765.
Humphrey extended the castle around its 16th century core to form a Palladian styled, four-storey tower with battlements, with new wings stretching away on three sides.
He also built a walled courtyard with hot houses beside the castle, and landscaped the island with two lakes and a large number of fir trees at a cost of £50,000.
His son, Charles Sturt, made the castle his primary home, although he was often living elsewhere as a result of his involvement in the Napoleonic Wars.
The castle was garrisoned throughout most of the 17th century. In the civil war of the 1640s between the supporters of Charles I and Parliament, the castle was held for Parliament, under the control of the Governor of Poole. The castle was refortified and in 1644, Parliament ordered that four pieces of artillery and four chests of muskets be sent to the castle, which by 1646 had a garrison of 20 men. During the Interregnum, the wealthy merchant Sir Robert Clayton bought the surrounding island. Clayton probably did not live in the castle, however, and it fell into disrepair; by the end of the century the town of Poole refused to garrison the decaying defences.
The amateur architect William Benson bought the island from Clayton's heirs in 1726 for £300. Benson set about converting the castle into a private residence, amid complaints from the authorities in Poole. The town took the matter to the Attorney General, where they argued that Benson had not bought the rights to the castle itself, only the island, and that the castle was a national fortification, originally built by Henry VIII and owned by Poole. Benson argued that the building had not originally been a castle but rather a lodging house; it had not been built by Henry and therefore the Crown had no particular rights over it - rather, the previous owners of the island had simply allowed the town and the government to place artillery there. The matter was eventually dropped and Benson demolished the external fortifications, created a Great Hall and planted trees and rare plants around the island.
The castle was sold to a Mr Chamberlayne and then onto Sir Gerard Sturt in 1762 and Gerard's cousin, Sir Humphrey Sturt, in 1765. Humphrey extended the castle around its 16th century core to form a Palladian styled, four-storey tower with battlements, with new wings stretching away on three sides. He also built a walled courtyard with hot houses beside the castle, and landscaped the island with two lakes and a large number of fir trees at a cost of £50,000. His son, Charles Sturt, made the castle his primary home, although he was often living elsewhere as a result of his involvement in the Napoleonic Wars.
Brownsea Castle