It's not generally known that in the Medieval ages a French army successfully invaded England, there were bloody sieges of Dover and Windsor, a guerrilla war in the Kent countryside and a French prince was proclaimed King of England in London.
It was all, unwittingly, due to King John, like so much else from Magna Carta to the legend of Robin Hood. John had signed Magna Carta in June 1215 but characteristically ignored it. Frustrated, the Barons rose in revolt, the "First Barons War". Bizarrely, they were legally entitled to do this by a clause in Magna Carta, clause 61, which allowed the barons to override the King if he broke the agreement, basically an application of the principle that unpaid debts can be recovered by force.
Anyway, the situation soon got past issues of Magna Carta, and it was clear John had to go.
In desperation, the barons offered the crown to Prince Louis of the House of Capet, the ambitious heir to the French crown. He enthusiastically agreed. In May 1216 Louis's army landed in Kent, captured the Cinque Ports, but not Dover, and headed for London. John bolted to Winchester, and Louis entered London virtually unopposed. Partly due to Johns unpopularity, partly sensing which way the wind was blowing, many nobles came to pay homage, and Louis was proclaimed king of England. Even the Scottish King Alexander II came down to acknowledge him, although he did annex Carlise on the way, which presumably belonged to Louis now.
Anyway, although John still had a significant army in the field, Louis soon controlled much of England. The only significant castle holding out against him was Dover, which he had bypassed in the invasion, and which lay on one of his main supply routes.
The siege of Dover
Dover is the closest port to France, and has always been strategically important. Although the castle was not as large as nowadays, it was still pretty impressive, with the keep and inner and outer bailey walls completed. A barbican and two towers protected the gate.
If Louis was lucky to have John as King of England as an opponent, that was balanced by John's generals, some of whom were remarkably competent men. Dover was under the command of the 56 year old Hubert de Burgh, who seems to have been an inspiring, and innovative leader. Hubert was of the de Burgh family from Norfolk and had been part of John's household from an early age. He obviously impressed as he had been made High Sheriff of Dorset, Somerset and Berkshire over the years, and castillan of various castles on the Welsh Marshes and Wallingford in Berkshire. He is destined to play a large part in this story.
Dover was one of the strongest castles in England, with a strong garrison of 140 knights, many men at arms and plentiful supplies. Louis returned on July 19th and lay siege, billeting his troops in and around the town. Presumably, many of his men were quartered in the fishing village on the east side of the river Dour, nearest the castle, the rest in the better accomadation in the town on the west bank, or local manor houses. He did the right things, seizing high ground to the north and using perriers and mangonels (trebuchets and catapults) to bombard the walls while he undermined the barbican defending the gate with miners. Perriers are a fairly primitive form of trebuchet, basically a high seesaw with a long beam on one side from which hung a sling contatining the rock, and a short beam on the other side from which hung ropes. When the crew, usuually 20-40 men, pulled on the ropes the rock was flung up into the air. This idea is not especially powerful, and also has a range similar to archery, so the crew needed protection or shielding. It's not clear exactly what was meant by a mangonel, as the term meant different things at different times.
Siege engines like this weren't just knocked together on the spot. For instance, for the siege of Bytham in Lincolnshire just 7 years later, 12 carts were used to transport artillery from London in sections. Whether Louis used the Royal stores in London, or had access to his fathers stores in France, which might have been more handy for Dover, is not clear.
Mining, though slow, was more effective against such a strong castle as Dover. It was simply a tunnel or even a cavity, shored up with beams, dug not so much to enter the castle as to arrive under the walls. This would then be deliberately collapsed, or exploded, and the wall would fall - obviously someone had to really know what they were doing! In this case someone did, the barbican fell and miners were sent on to topple the two towers of the gate itself. When one tower fell the castle was stormed. Here things started to go wrong, as de Burgh's men drove off the attack, and blocked the breach with timbers taken from castle buildings, and Louis was back where he started.
Meanwhile, Kent was the battlefield for a guerrilla war....
"A certain youth, William by name, a fighter and a loyalist [to King John] who despised those who were not, gathered a vast number of archers in the forests and waste places [of the Kent and Sussex Weald], all of them men of the region, and all the time they attacked and disrupted the enemy, and as a result of their intense resistance many thousands of Frenchmen were slain". Roger of Wendover, Flores Historiarum, II. 182".
William´s origins are a bit of a mystery, he may even have been one of John´s many Flemish mercenaries, but he does seem to have been only about 21 at the time. Somehow he raised thousands of volunteers amonst the archers of the Weald in Kent and Sussex and harrased the French wherever he could. With most communications between London and Dover, or London and France, passing through Kent he was holding the jugular. William has probably suffered historically by starting on the wrong side, which is a pity as with this and his later exploits he makes Robin of Lockesley look very small beer.
We'll meet William of Cassingham and Hubert de Burgh again later, but for now Louis was thinking he didn't have time for all this. On October 14th he called a truce and marched back to London.
Death of John
On October 18th Louis had more bad news, John had died at Newark Castle. Now, the death was so convenient, for so many people, that rumours immediately started. It was poisoned ale, poisoned plums, Shakespere, writing centuries later, has a monk giving the poison. Frankly, its hard to say now how he died. He had apparently contracted dysentery in Lynn (now Kings Lynn) which might well have killed him. Anyway, his body was taken to Worcester where he was buried in the cathedral.
However it had occurred, Johns death changed the game completely. Opposing a cruel and despotic king was one thing, serving a foreign prince quite another. John had an heir, Henry, and attention now focused on him. Nine year old Henry was crowned in Gloucester on October 28th, 1216. They didn't have London, they didn't even have a crown, using a gold necklace, but they had the papal legate in attendence, and crucially, they had a figurehead.
We now meet the third of Louis's nemesi, William Marshall. William was already a major player. At 69 years old he had served Henry II and Richard the Lioneart, and then John. Starting as the younger son of a fairly minor noble, Marshall now was a major landowner in his own right and hugely respected. It was in large part Marshall who had kept John in the game as long as he had, and it was apparently Marshall who John entrusted with Henry´s care on his death bed. On November 11th 1216 the Kings Council appointed Marshall as Regent in Henry's name. The fight back had begun.
The sieges of Dover http://www.deremilitari.org/resources/articles/goodall.htm
A Note on William of Cassingham
G. R. Stephens
Speculum, Vol. 16, No. 2 (Apr., 1941), pp. 216-223
An online guide to trebuchets