The Battle of Lewes on 14 May 1264 was one of the two main battles of the Second Barons' War. Fought between the forces of King Henry III and the barons led by Simon de Montfort, it resulted in the defeat of the king and the signing of the Mise of Lewes.
Tired of bad government and royal extravagance, the barons wanted the country to be governed by a council rather than by the king.
The king's army camped here on 12th May, the eve of the feast of St Pancras, an important religious celebration for the Priory. The soldiers' presence caused much disruption for the monks.
The battle was fought to the north of Lewes and gave victory to Simon de Montfort. The king's army retreated to the Priory with de Montfort's forces in pursuit, their blazing arrows causing considerable damage to thatched roofs.
The king's defeat led to the Mise of Lewes (no copies of which have survived), a treaty which restricted the authority of the king.
The conflict divided the monks of the Priory and some were sent back to Cluny, others were punished at Lewes. During excavations to lay the railway line in 1845, a large burial ground was discovered at the Priory containing hundreds of bodies from the battle.
More information on these external links:
Battle of Lewes - Wikipedia
Battle of Lewes - UK Battlefields Resource Centre