Tresco also had an important harbour, so defences were made there as well. Kiligrew and his son again had the task of picking a site for King Charles? castle, built in 1550, (before Harry’s walls).

The site was poor. It did overlook the harbour and did cover a wide area but it was built on top of a hill and this did not help the range of the cannons as only one entrance to the harbour was covered. It also proved a difficult task to get the cannons pointing down at the right angle, and as the castle was built on a steep hill, as long as ships stayed close to the shore, the cannons would overshoot every time.

Though not tested in wartime, ?[It] can make no good shot so steep downwards,? said Francis Godolphin. It seems that the main factor that Kiligrew took into consideration when deciding the site for a new castle was the height of the land on which the castle was built.

Unfortunately, Kiligrew did not take other important features into account, such as geometry of the cannon fire which made Harry’s Walls and King Charles? Castle so unsuccessful. In contrast, one of the great success stories of fortifications in Scilly was Cromwell’s castle, built in 1651 as a replacement for King Charles?.