onisbrough Castle was a Norman castle belonging to William, Earl Warenne, son-in-law of William the Conqueror. The castle or area where it now stands appears to have belonged to the de Warenne family. William de Warenne was made Earl of Surrey in 1088, and was followed in the same year by his son after William's death in battle. The third Earl died in the Crusades in 1147 leaving no male heir. His daughter, Isabel, married King Stephen's son, William de Blois who became the fourth Earl. But again no male heir was produced. Isabel married Hamelin Plantagenet, Henry II's brother. It is Hamelin who built the stone keep as it appears now at Conisbrough. The castle was visited by Hamelin's nephew King John in March 1202.

The keep is the most interesting feature of the stronghold and projects slightly beyond the curtain wall near the north-east edge. The plan of the keep resembles a circle inscribed within a six sided star. The unusual shape shows how castle design evolved in response to changing methods of attack such as undermining. The six large butresses that give the keep its strength are solid apart from one that holds the chapel and staircases are built into the thickness of the towers core masonry. The entrance to the keep is on the first floor via a modern staircase.

The bailey is roughly semi-circular in shape with the keep to the north-east and the gatehouse to the south. The low remains of the other buildings can be seen along the west and north sides of the curtain wall.