Architectural historian Anthony Emery thinks Piel was built in three nervous phases. The central keep came first, almost a fortified summer house for the abbot's use. Then, as Scottish raids worsened and the crenellation licence loosened the church's hand, the inner bailey wall went up to wrap the keep in stone. Finally, an outer bailey followed, a belt-and-braces job for a coastline that never quite felt safe. Most castles speak in the language of kings. Piel speaks the language of accountants and worried abbots. Every wall here was a hedge against the next raid, the next wool seizure, the next clutching financial year. By 1408 a new abbot, John Bolton, decided the upkeep was crushing him and tried to pull the defences down. Henry IV refused permission. The walls stayed up.

Officially, English wool in the 14th and 15th centuries could only be sold legally through Calais. Unofficially, plenty of it went out through Piel. Merchants in Calais wrote angry letters about Furness monks moving fleeces through their island fortress and bypassing the staple altogether. The Crown was supposed to take a cut of every legal sale, so this was tax fraud with cloisters attached. The same harbour the castle was built to defend became a back door for the abbey's own contraband. Two centuries later the smuggling tradition outlived the monks; in the 18th century the Crown stationed customs officers and harbour pilots on Piel to try, with limited success, to keep the trade honest.

In June 1487, a ten-year-old boy named Lambert Simnel stepped onto Piel Island claiming to be the Earl of Warwick and the rightful king of England. He was almost certainly a baker's son coached by a priest, but two thousand German mercenaries and four thousand Irish soldiers came with him, and a crown of sorts went with them. Simnel held court at Piel Castle for a few days while local Lancastrian gentry quietly decided whether to come out for him. Most did not. He marched inland and was defeated at the Battle of Stoke Field on 16 June, the last battle of the Wars of the Roses. Henry VII pardoned the boy and put him to work in the royal kitchens. The castle that had hosted his improbable coronation was already drifting toward ruin. By 1534 a survey called it sore decayed.

After the Dissolution in 1537, Piel passed to the Crown, then to the Duke of Albemarle after 1660, then on to the Dukes of Buccleuch. In 1805 the painter Sir George Beaumont set up his easel and made Peele Castle in a Storm, all crashing wave and black weather. William Wordsworth saw the picture, remembered his own time at Rampside looking out at the island, and in 1806 wrote his Elegiac Stanzas, an elegy to his brother John, drowned at sea. The castle became Romantic property, an emblem of human work standing against ungovernable water. By the 1870s the Duke of Buccleuch was paying for serious restoration and outworks to hold off the tide. In 1920 the family handed the whole island to the people of Barrow-in-Furness as a First World War memorial.

Today Piel Castle is in the care of English Heritage, and access is free once you reach the island by ferry from Roa Island or by walking the sands from Walney at low tide. The Guardian listed Piel in 2022 among the English castles most at risk from coastal erosion. The same beach that supplied John Cockerham's masons is steadily clawing the walls back. Visit the ruined three-storey keep, look up at empty windows that once housed a worried abbot, and walk down for a pint at the Ship Inn, where the landlord still holds the courtesy title of King of Piel - a joke that began with Lambert Simnel and is now older than the United States.