Despite its name King Charles's Castle actually predates Harry's Walls on St Mary's by three or four years.
Like the Blockhouse on the other side of Tresco, it was begun in 1548 in the reign of the boy king Edward VI, and it's not a castle more of an artillery fort.
As the plan on the English Heritage Display Board shows, the fort consisted of a polygonal gun room, originally with five gun embrasures (one of which was later converted into a room), a hall/kitchen and two small sleeping chambers. It may have had an upper floor, although there is no sign of a staircase.
Later, probably around 1627, although possibly during the Civil War, a low bank and ditch with two bastions was built to defend the fort from landward attack.
It was largely superseded by Cromwell's Castle, built around 1652, on the shore below, but remained in use as soldiers lodgings for some years.