The oldest fabric here dates to between 1170 and 1190, when an Augustinian hospital known as the Spital cared for the sick and the destitute on the High Street. After the Reformation gutted such institutions, the town corporation took the building and turned it into a free grammar school - which is where Oliver Cromwell, born in Huntingdon in 1599, learned the rhetoric and theology he would later wield against bishops and kings. Pepys passed through the same desks decades later. By 1863 the medieval hall was crumbling; in 1878 the architect Robert Hutchinson restored it for £900, paid for by the playwright Dion Boucicault in memory of his son, killed two years earlier in the Abbots Ripton rail accident. The building survives today as five decorative arches on a west front, two surviving bays of a vanished nave, and a bellcote pointing at the Cambridgeshire sky.

The collection - founded in 1962 and judged the finest body of Cromwelliana in the United Kingdom - is the kind of thing that makes historians whisper. There are portraits painted from life and death masks cast within hours of his passing in 1658. There is his hat, his pocket watch, items of 17th-century military equipment on loan from the Royal Armouries, and a copy of the Lord Protector medal struck by the engraver Thomas Simon. About 70 percent of the objects belong to the museum outright; the rest arrive on loan from descendants of Henry Cromwell, his fourth son, and from the Museum of London. Threaded through the cases is the Tangye Collection - assembled by the Victorian engineer Richard Tangye, whose admiration for Cromwell was unfashionable enough to be a kind of devotion.

How do you curate a man whose name still divides a country? Cromwell ended monarchy, reformed Parliament, sheltered Jews who had been expelled from England for centuries, and presided over a brutal Irish campaign whose memory still inflames opinion in Drogheda and Wexford. The museum does not look away. It presents the regicide, the Protectorate, the dissolution of Parliament, the slaughter, alongside the Bible-reading farmer who never wanted to be king. Pilgrims of one persuasion arrive expecting a hero; pilgrims of another arrive ready to condemn. The collection invites both to slow down and look at what the man actually wore, signed, fought with, prayed over.

The museum has been threatened more than once. In 2013, Cambridgeshire County Council proposed closing it as a budget measure, citing a saving of £20,000 a year. The town pushed back, hard. Letters were written, petitions signed, the BBC dispatched cameras. On 1 April 2016 management passed from the council to a charitable trust dedicated specifically to Cromwell's legacy. The building that had been hospital, then schoolroom, then museum had survived its latest threat - rescued, fittingly, by the same kind of stubborn civic feeling that produced its most famous pupil in the first place.