After the defeat of the Spanish Armada in 1588, the threat from Spain remained real, and the Crown continued to fortify the country's coasts. Scilly's Governor, Sir Francis Godolphin, oversaw the building of Star Castle in 1593, along with an impressive set of curtain walls now known as Garrison Walls. The fortress on St Mary's is now a hotel (although the Garrison Walls are managed by English Heritage).
Why "Star Castle"? If you look at the site from above, you'll see that its (then) innovative angled bastions create a star-shaped outline. It was a fearsome defensive structure; however, the anticipated invasion from Spain never came.
The castle and walls saw plenty of action during the Civil War. It was originally a Royalist stronghold (it's one of the many places where Prince Charles stayed en route to exile), then after its capture, was used by the Parliamentarians as a prison. Star Castle remained a garrison until the mid 18th-century, when it became the Governor's residence. Unlike many other Scilly defences, it wasn't called up in WWII: Star Castle became a hotel in 1933.
If you'd like to see the former keep and walls up close, non-residents are welcome to eat and drink at the Star Castle Hotel. You can enjoy lunch or drinks in the 400-year old Dungeon Bar or take afternoon tea on the ramparts. Today's dining room is housed in the former officers' mess, and even the guards' rooms have been converted into guest accommodation.