The site already had a small sixteenth-century blockhouse on it - a low fortification at the water's edge beneath the failed King Charles's Castle uphill. Between 1651 and 1652 Sir Robert Blake had a three-storey circular tower built on top of this blockhouse, 13.45 metres across and 15.1 metres high, with walls nearly thirteen feet thick made of massive rubble. A later eighteenth-century survey would describe the tower as 'a Huge Mass of Masonry,' which is what you build when you have a lot of stone, no shortage of labour, and an Atlantic gale rattling your scaffolding. Some of the stone came directly from the ruins of King Charles's Castle - the Royalists' own building material recycled into Parliament's reply. Six gun-ports with broad external splays on the roof allowed cannon to cover the channel between Bryher and Tresco, and there may have been an additional gun platform just below the tower.

Why build a new fort at all? The Parliamentary forces who took the Scillies in 1651 were not primarily worried about a Royalist counter-attack. They were worried about the Dutch. England and the Netherlands were drifting toward what would become the First Anglo-Dutch War of 1652-1654, and Royalist privateers based on Tresco had been preying on Dutch shipping in the western approaches for years. The Dutch had threatened to intervene directly to suppress the privateers, and Parliament feared that any Dutch foothold in the Scillies could become a permanent foreign base just thirty miles off Cornwall. Cromwell's Castle was built specifically to guard the deep-water entrance to New Grimsby harbour - a channel that allowed vessels to slip through to the inner anchorages of the archipelago. With the new tower commanding that approach at sea level, no fleet could simply sail past it the way Blake himself had sailed past King Charles's Castle from below.

After the Restoration of Charles II in 1660 the castle was surveyed and patched up. By the 1730s it had ten artillery positions on paper but not nearly enough working guns to fill them. Then in 1739 the War of Jenkins' Ear broke out between Britain and Spain - a conflict named after an English captain who claimed Spanish coastguards had sliced off his ear and pickled it. The Master Gunner at the Garrison on St Mary's, Abraham Tovey, was sent over to upgrade the defences. Tovey built a large gun platform for six guns on the southwest side of the tower, possibly on top of any pre-existing sixteenth-century platform. A parapet protected the gunners, a new entrance was cut into the main tower, and other adjustments followed. When the antiquary William Borlase visited in 1752, the platform carried 9-pounder cannon and the tower roof carried 4-pounders - but the place was already unmanned and decaying.

By 1922 the lease had passed to Arthur Dorrien-Smith, the Tresco landowner, who handed several Tresco properties - including the castle - into the guardianship of the Ministry of Works. English Heritage took over in due course, and today Cromwell's Castle is open to visitors. The interior wooden floors are long gone, but the spiral staircase still climbs through the thickness of the wall to the vaulted stone roof, and from the parapet you can still look out through the gun-ports across the channel to Bryher. The castle is a Grade II* listed building - the second-highest level of protection English law offers - and protected as a scheduled monument. It is also, importantly, one of the relatively few surviving structures built during the eleven years when England was a republic. The Restoration restored Charles II but left this round granite drum standing as a quiet reminder of the other path the country might have taken.