Yarmouth 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.

[1] 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.

[6] The town of Yarmouth on the Isle of Wight may have been attacked by the French in 1543; if so, this raid probably encouraged the construction of a castle there as part of the second wave of Device Forts.

[7][a] The fort functioned alongside the existing defences in the Solent and protected the main crossing from the west side of the island to the mainland.

[9] Yarmouth Castle was a square artillery fort built around a central courtyard with an angular, "arrow-head" bastion protecting the landward side.

[10] It was initially equipped with three cannons and culverins, and twelve smaller guns, firing from a line of embrasures along the seaward side of the castle.

[12] The castle was constructed by George Mills under the direction of Richard Worsley, the Captain of the Island, on land belonging to the Crown, possibly on the site of a church destroyed during the events of 1543.

Worsley was dismissed in favour of a Roman Catholic appointee in 1553 and Udall was executed in 1555 for his role in the Dudley conspiracy to overthrow the Queen.

[23] Suggestions that a half-moon battery should be added to the defences were not progressed, but in 1632 the parapets were raised in height and further lodgings and a long room for stores were constructed within the castle.

Initially, Captain Barnaby Burley, a relative of John, and an ardent Royalist, held the castle on behalf of the King with a tiny garrison.

[23] Early during the Interregnum it was decided to increase the size of the garrison at the castle from 30 to 70 soldiers, due to concerns about a potential Royalist attack from the island of Jersey.

[14][b] When Charles II returned to the throne in 1660, he demobilised most of the existing army and the following year the garrison at Yarmouth was given four days notice to leave the castle.

[26] The King announced that the castle's artillery would be sent to Cowes, unless the town of Yarmouth agreed to take over the financial responsibility of running the site themselves.

[31] By the 18th century, however, Yarmouth Harbour had gradually silted up and been destroyed by industrial developments, reducing the value of the anchorage, and the design of the castle had become outdated.

[47] Earlier Henrician castles had used the older European style of semi-circular bastions to avoid presenting any weak spots in the stonework, but an arrow-headed design enabled defenders to provide much more effective supporting fire against an attacking force.

[49] On the ground floor, the entrance to the castle leads through to a courtyard, linked to four barrel-vaulted rooms in the south-west corner, originally 17th-century lodgings for the garrison.