Then Louis received the worst possible news - John had died. Support started to crystallise around Johns young heir, Henry III, encouraged by the highly capable and respected William Marshall. Louis went on the offensive, taking Hertford and Berkhamsted castles in December 1216, but it was soon clear he would need reinforcements. Early in 1217 he set off for France - or tried to.
William Marshall. Actually more a symbolic representation, but it does show a typical knight of the time. From the Historia Major of Matthew Paris
Lewes to Winchelsea
William of Cassingham, who had been leading a guerrilla war against Louis in the Weald, ambushed Louis at Lewes in Sussex, routed his troops and sent them running for the coast. Stragglers were picked off and Louis´s army blocked and channelled by destroyed bridges until they reached Winchelsea, which was exactly where Cassingham wanted them. The men of Winchelsea had burnt their mills and left by boat for Marshall´s army under Philip of Aubigny in Rye, Louis was trapped, hemmed in by the sea, his communications with London broken and ships from the Cinque Ports blockading the coast. Fortunately for him a French fleet managed to break through before his forces were starved into surrender and he was evacuated to France. Louis ran with his tail between his legs, but he wasn´t finished yet.
The 2nd Siege of Dover
Reinforced with a new French army Louis set off across the channel. His plan was to land at Dover and launch his troops against the castle. However, the French siege camp was attacked and destroyed by Cassingham and an illegitimate son of John´s, Oliver, and Louis had to land along the coast at Sandwich. From here he marched overland to Dover and started a siege in earnest, but the garrison under the redoubtable Herbert de Burgh again held out. So many troops were required for the siege, and to keep open communications with London, that they were diverted from other theatres of the war. This opened opportunities
The Battle of Lincoln
William Marshall had skillfully amassed a sizable army, and he now felt able to challenge Louis in the field. Or rather, Thomas, the Count of Perche. Perche was in command of Louis´s largest field force, but he also had his own agenda, hoping to recover the ancestral lands including Newbury in Berkshire and Toddington in Buckinghamshire that John had taken from his family. At this moment he was commanding a mixed English/French force besieging Lincoln, or rather Lincoln castle, the city having already fallen. Marshall assembled in Newark, with 400 knights, an unknown number of foot and mounted troops, and 250 crossbowmen under Falkes de Breaute, a somewhat contraversial figure. He was well named, a John loyalist already responsible for the sacking of Worcester and St Albans, but he was capable.
Marshall approached from the south west, his army in seven divisions, the crossbowmen about a mile in front, the baggage train well behind. His men were well fed and rested, and according to an account by the monk Roger of Wendover, inspired by the presence of the Papal Legate.
Meanwhile Perche dithered what to do. The English under Robert Fitzwalter wanted to attack, whilst the French were more cautious, according to Wendover because they mistook the baggage train of another army. In the end, Perche decided to hold Marshall at the city walls and press the siege, which gave the initiative to Marshall, a big mistake. De Breaute's crossbowmen stormed the North Gate and then took up positions on the roof tops over looking Perche's troops and poured fire into them. Armour at this time was mainly mail, and a heavy shield, which might give some protection from crossbow bolts, but Breuate's men were aiming for the horses. Knights were sent tumbling to the ground to be picked off, or captured for ransom. Virtually immune from counterattack, the crossbows decimated the besieging army, even before Marshall charged in with his knights and infantry. It was a complete success, Perche killed and most of Louis's commanders captured. A success marred though by the looting of Lincoln, the so called "Lincoln Fair", but nonetheless Louis's army was sent streaming back to London, with many of the French troops being ambushed and killed on the way.
William now felt able to march on London, but it wasn´t to prove necessary. Louis was now relying completely on his links with France, which meant control of the Channel. This was about to be challenged.
An account of William of Cassingham's life can be found in "A Note on William of Cassingham" by GR Stephens (Speculum vol 16, p216, 1942).