Montfort then marched southwards from the capital.  Besides the baronial retinues, a swarm of Londoners, eager for the fray, though unaccustomed to military restraints, accompanied him.  On May 13 he encamped at Fletching, a village hidden among the dense oak woods of the Weald, some nine miles north of Lewes.  A last effort of diplomacy was attempted by Bishop Cantilupe of Worcester who, despite papal censures, still accompanied the baronial forces.  But the royalists would not listen to the mediation of so pronounced a partisan.  Nothing therefore was left but the appeal to the sword.

The royal army was the more numerous, and included the greater names.  Of the heroes of the struggle of 1258 the majority was in the king’s camp, including most of the lords of the Welsh march, and the hardly less fierce barons of the north, whose grandfathers had wrested the Great Charter from John.  The returned Poitevins with their followers mustered strongly, and the confidence of the royalists was so great that they neglected all military preparations.  The poverty of Montfort’s host in historic families attested the complete disintegration of the party since 1263.  Its strength lay in the young enthusiasts, who were still dominated by the strong personality and generous ideals of Leicester, such as the Earl of Gloucester, or Humphrey Bohun of Brecon, whose father, the Earl of Hereford, was fighting upon the king’s side.  Early on the morning of May 14 Montfort arrayed his troops and marched southward in the direction of Lewes.  Dawn had hardly broken when the troops were massed on the summit of the South Downs, overlooking Lewes from the north-west.

Lewes is situated on the right bank of a great curve of the river Ouse, which almost encircles the town.  To the south are the low-lying marshes through which the river meanders towards the sea, while to the north, east, and west are the bare slopes of the South Downs, through which the river forces its way past the gap in which the town is situated.  To the north of the town lies the strong castle of the Warennes, wherein Edward had taken up his quarters, while in the southern suburb the Cluniac priory of St. Pancras, the chief foundation of the Warennes, afforded lodgings for King Henry and the King of the Romans.  When Simon reached the summit of the downs, his movements were visible from the walls.  But the royal army was still sleeping and its sentinels kept such bad watch that the earl was able to array his troops at his leisure.

From the summit of the hills two great spurs, separated by a waterless valley, slope down towards the north and west sides of the town.  The more northerly led straight to the castle, and the more southerly to the priory.  Montfort’s plan was to throw his main strength on the attack on the priory, while deluding the enemy into the belief that his chief object was to attack the castle.  He was not yet fully recovered from his fall from his horse, and