Just north of the town centre, beyond the railway line, sit the surprisingly large earthworks of Berkhamsted Castle — a Norman motte-and-bailey thrown up within months of the Conquest and rebuilt in stone over the following two centuries. The high motte still rises behind two concentric water-filled moats; the flint walls of the bailey keep stand to between one and three metres around most of the perimeter.
The castle was a royal residence through the Middle Ages, owned at different times by Thomas Becket, Edward the Black Prince, and Cicely Neville (mother of Edward IV). It fell out of use by the 1500s and was already a ruin when the railway sliced past its northern edge in the 1830s. Entry is free; English Heritage information boards walk you through the layout.