Berkhamsted Castle's strong defences held out in a two-week long siege.

View of the defensive ditch and ruins of the castle © English Heritage Photo Library Berkhamsted was designed as a fortress, with impressive earthwork defences, high motte and stone curtain wall. The water-filled ditches prevented tunnelling under the wall, and the motte could protect the bailey as well as defend attacks from the north.

In 1216, the castle's defences were put to the test. Prince Louis of France had invaded England at the invitation of the English barons, who were opposed to King John. When John died in October 1216, his nine year old son was crowned King Henry III. Louis had to act quickly. A plan of the earthwork defences © English Heritage On 6 December 1216 he took Hertford Castle, and moved on to Berkhamsted.

Louis besieged the castle for two weeks, battering it with huge stones flung from siege weapons. The earthwork buttresses by the outer ditch were probably platforms to support them, although it is still unclear which side built them. The castle surrendered finally on the orders of the king.