Thunderfield Castle is an earth early 12th century ringwork and bailey or a fortified manor house, founded by the Clare family. Guarding the oval platform are two wet ditches and to the north is a large bailey with a small D-shaped inner barbican, both with their own wet ditches. The ditches were originally dry but the site was greatly remodelled in the Victorian era, with alterations to the ditches and a system of pipes and valves installed for the supply of water. The later barbican which could be mistaken for a motte, is at a higher level and an early 13th century outer ditch, formed a small outer bailey beyond the barbican. Excavations in 1936, found the hearth of a bloomery and 13th to 15th century pottery. With the dense cover of trees, the site is best viewed in the winter. 6 miles north is Reigate Castle and 14 miles east is Hever Castle.