Star Castle is a fortress on St Mary's, Isles of Scilly, built in 1593 by Robert Adams, Surveyor of the Royal Works (d.1595) and Francis Godolphin, Captain of the Scilly Isles, during the "Spanish invasion scare."

The Star Castle was built in 1593 by the Surveyor of the Royal Works and mapmaker Robert Adams, under the direction of Francis Godolphin, Captain of the Scilly Isles, following the Spanish Armada of 1588.

[1] Fearing another Spanish invasion, in May 1593 Queen Elizabeth I ordered the construction of a fort and two sconces as a lookout for any intruder ships.

It is at the centre of a fortification system around the west side of St Mary’s known as the Garrison.

It comprises an outer wall around the outcrop of Hew Hill, protecting the town and the castle, with strategically placed gun batteries at regular intervals around the outer wall, allowing covering fire at all angles.