King Charles's Castle is a ruined artillery fort overlooking New Grimsby harbour on the island of Tresco in the Isles of Scilly.

Built between 1548 and 1551 to protect the islands from French attack, it would have held a battery of guns and an accompanying garrison, designed to prevent enemy vessels from entering the harbour.

The castle is polygonal in design, constructed from granite stone, with the gun battery at the front, and a dining room, kitchen and living accommodation at the rear.

To mitigate this, an additional blockhouse was probably constructed below, closer to the water, but eventually a new fortification, the Star Castle, was built instead on the neighbouring island of St Mary's, which became the main fort in the Scilly Isles.

In the aftermath of the English Civil War, the Scilly Isles were held by the Royalist sympathisers of King Charles I, who gave the castle its current name.

[2] Henry initially responded by fortifying the coasts of England, constructing new artillery forts designed to defend against the longer-range cannons that were becoming common in the 16th century.

[3] Flemming was supported in this effort with a shipment of lead to aid in construction, and money raised from the dissolution of the monasteries in England.

[3] The building work initially focused on the island of Tresco and was carried out under the direction of John Killigrew, the captain of Pendennis Castle in Falmouth.

[7] Edward Seymour fell from power in 1549, and after his removal a new survey was carried out by Captain William Tyrell, in part to confirm that the new fortifications were suitably located across the islands.

[9] The Old Blockhouse appears to have been completed, but the Crown's resources had become badly stretched and it was decided at the end of 1552 to curtail further expenditure on the Scilly Isles.

[11][nb 1] Edward's successor, Queen Mary I, intended to establish a garrison of 150 soldiers on the islands, but it is uncertain if these numbers were ever achieved.

[15] Artillery pieces in the 16th century could not fire in this position as the cannonballs would fall out of their muzzles, making the castle much less effective in defending Tresco.

[31] The antiquary William Borlase was critical of the castle—by then in ruins—during his 1752 visit, describing it as a "work of labour and expense, rather than of skill", and attributing its abandonment to its poor military utility.

[32] The writer John Troutbeck, commenting at the end of the century, took a similar perspective, praising the thickness of the walls, but noting that it was ill-positioned to fire into the harbour, unlike its replacement below.

[35] King Charles's Castle is a polygonal stone building, composed of a gun battery on one side and living quarters on the other.

[36] The castle is made from dressed and rubble granite, incorporating an underling rocky outcrop into the design in the north-west and south-west corners.