As the nearest sea crossing point to France, the castle at Dover has been a site of strategic military importance since Roman times.
It is interesting to note that Matthew Paris, the medieval chronicler, supported this assertion when he referred to Dover as "Clavis Angliae" (the key of England).
The castle's colourful history really started in 1216 when King John was embroiled in a civil war with a large group of rebel barons who had offered the throne of England to Prince Louis of France in the previous year. Rebels had set up their base in London and were awaiting the arrival of the French prince in order to deliver the final crushing blow to King John's hopes of retaining his crown.
Louis landed in England on May the 21st 1216, forcing John to flee north with his army while the French Prince marched to Canterbury where he was welcomed by the citizens who threw open the gates for him. Following this easy success, the city of Rochester was quickly taken and, after a short siege, so was the castle enabling Louis's army to control the river crossing and the road to London.
Prospects were now looking bleak for John as the French army swarmed into London and secured the city. After less than a month, Winchester had also fallen. Swiftly and decisively, Louis had taken control of southern England and, with victory and the throne in sight, he turned his attention towards Dover.
With his army now encamped in the town of Dover, Louis spent several days observing the castle and the surrounding area, so that

by mid-July he was prepared to lay siege to the mighty fortress.
Leaving half of his army to protect the town, the twenty-nine year old prince set up an encampment directly in front of the castle, siege engines in the form of mangonels and perriers were erected along with a wattle siege tower and, by sending his fleet to sea, he had by now cut off any possible route for supplies or reinforcements from land or sea.
Dover castle was now on it's own and at the mercy of the French invaders.
Miners were instructed to tunnel under the timber palisade of the barbican which was soon breached and captured. Moving on to the castle gates, undermining was once again employed and succeeded in bringing down one of the two massive stone towers. Through the breach in the wall, the French army stormed the castle but were held back by the castle's defenders who had managed to close the gap using timbers and enormous cross-beams which had been stripped from the castle's interior.
Defending their positions staunchly, the garrison forced Louis to pull back his army. With this heroic defence of the castle coupled with the fact that the French siege artillery had failed to make any impression on the castle walls, Louis decided to negotiate a truce.
On the 14th of October 1216 a truce was agreed but four days later King John died leaving only his nine year old son Henry (later to become King Henry III) as his heir.