Conisbrough Castle is a medieval fortification in Conisbrough, South Yorkshire, England. The castle was initially built in the 11th century by William de Warenne, the Earl of Surrey, after the Norman conquest of England in 1066. Hamelin Plantagenet, the illegitimate, parvenu son of Henry II, acquired the property by marriage in the late 12th century. Hamelin and his son William rebuilt the castle in stone, including its prominent 28-metre (92 ft)-high keep. The castle remained in the family line into the 14th century, despite being seized several times by the Crown.