Kenilworth's great mere was a feat of medieval hydraulic engineering as much as a feat of defensive design. A dam to the south of the castle held back a wide artificial lake that flooded the ground on three sides. The single approach lay across a long causeway from the south, defensible to a degree few castles in England could match. Ditches and a second pool covered the remaining flank. To take Kenilworth a besieger had to come at the walls either across that single narrow causeway, where every step was within bowshot, or across water. Henry III tried both. He had spent the previous autumn trying to contain the garrison and force terms; by June he had concluded that nothing short of full siege would do.

What followed was the largest siege ever undertaken in England. The royal forces brought stone-throwing engines, presumably trebuchets, and turres ligneas - wooden towers. The chronicler William Rishanger described an enormous tower built at the order of Edmund, the king's younger son, costly, of astonishing height and width, ingeniously placed against the wall, with internal compartments holding two hundred crossbowmen or more so that they might shoot down on the defenders within the walls. The garrison answered it with a single mangonel, a torsion-powered stone-throwing engine erected inside the besieged area, which battered the great tower with such persistence that, in time, it broke and shattered. Another royal engine, called the Bear because of its size, also housed archers; the garrison's stone-thrower struck back with such force that, Rishanger wrote, for a long time the Bear lost its ability to deliver and inflict damage. Barges were sent overland from Chester to attempt an assault across the lake. That, too, failed.

What no medieval garrison could defeat was time. A castle defends not just walls and gates but the men inside them, and men need food. Twelve hundred defenders is a large household; Kenilworth's stores had not been laid in for a siege of indefinite length. As summer turned to autumn and autumn to winter the bread ran out, the salted meat ran low, and disease - dysentery, almost certainly, or something like it in the unsanitary press of so many men in confined quarters - began to thin the ranks. Archbishop William Freney was sent by the king to negotiate; the garrison refused him entry. Their position was hardening even as their bodies weakened. But by December they could hold no longer. The garrison surrendered on 13 December 1266, on terms.

Those terms were the Dictum of Kenilworth, drafted by a commission of bishops and barons in October while the siege still ground on. It was a remarkable document for its time - a negotiated peace, not a punitive settlement. The Disinherited were not to lose their lands permanently. They could buy them back, at multiples of their annual value, from the royalist supporters who had been granted them after Evesham: typically five times the annual income for the most active rebels, less for those who had played minor parts. The Dictum reaffirmed Henry's authority but left the rebels a path back into society. It was a deliberate act of reconciliation in a country exhausted by civil war. Four chapters of the Dictum were later incorporated into the Statute of Marlborough of 1267, and remarkably, those chapters remain in force in English law to this day - among the very oldest pieces of English statutory law still on the books.

Kenilworth Castle continued to be a great fortress for centuries after 1266. Edward I expanded it; John of Gaunt rebuilt much of the residential range; Robert Dudley, Earl of Leicester, transformed it for Queen Elizabeth in the 1570s. It was finally slighted by Parliamentary forces after the English Civil War in the 1640s, when the great mere was drained and the keep partially demolished to ensure no royalist could ever hold it again. The red sandstone ruins are open today, managed by English Heritage, and the dry depression where the lake once gleamed can still be traced in the ground. Archaeologists found catapult-fired stones at the site in 2024, almost certainly fired at the walls during those six months of 1266. The walls held. The men behind them simply ran out of food.