Robert, Count of Mortain and Earl of Cornwall probably founded Berkhansted Castle. It was certainly held by him at the time of the Domesday survey. As William I's half-brother, Robert did well for himself out of the Norman Conquest, but his son made the mistake of supporting Robert of Normandy against Henry I. As a result, the Crown confiscated the castle. During the twelfth century it was leased to certain individuals, including Thomas Becket.
The castle is a classic example of a motte and bailey stronghold, even if roads and railway have gnawed at its edges. The motte is tall and conical, and a double ditch surrounds the bailey with a rampart in between. Until the 1950s, the inner ditch remained full of water.
In front of the outer ditch, on the north and east sides, following the circumference of the motte, rises a strong rampart. It is probably a concentric defense provided by Richard of Cornwall, though it has been suggested that the earth bastions that project from it could have been raised as platforms for treuchets during the Dauphin Louis' siege.
The shell keep, which crowned the motte, has vanished but there are remains of the walls that descended to join the bailey curtain. Considerable lengths of this flint curtain survive, especially on the east side. At least some of the masonry dates from the time when Thomas Becket occupied the castle, though the money came from Henry II's executor.
Three semi-circular towers flanked the curtain, and if they date from Becket's tenure they are remarkably early. Little more than foundations are left of the towers now. The stump of a large oblong structure on the west curtain is probably the tower built by Richard of Cornwall in 1254. Foundations show that the north end of the bailey was walled off to form a separate enclosure, in effect a barbican in front of the motte.