At the heart of the Country Park is Clare Castle, first built in the 11th century by Richard FitzGilbert, a Norman knight who was rewarded by William the Conqueror with large estates in Suffolk, Essex and Kent. He is estimated to have been the sixth wealthiest non-royal layman in England at the time of Domesday Book in 1086. Richard had built a castle at Tonbridge before the one in Clare, but Richard and his descendants made Clare their administrative seat, eventually taking the family name from the town. The Castle flourished for 3 centuries until the death of Elizabeth de Clare in 1360. The marriage of her granddaughter led the Castle into royal hands and by the sixteenth century it was already in ruins. Later it passed into private hands resulting eventually in it being compulsorily purchased to form part of the Great Eastern Railway as Clare station which was open from 1865 until 1967. In 1970, the remaining Castle grounds were donated to Clare Rural District Council and West Suffolk Council, the railway land was re-purchased, and in 1972 Clare Castle Country Park opened, by then solely owned by Suffolk County Council.
Clare Castle has a motte and bailey structure. The motte at about 60 feet high is one of the highest in the country, once surmounted by a shell keep of at least two stories, over 50 feet in diameter,of which now only a portion remains with its buttresses. The keep, made of flint, was probably constructed in the thirteenth century replacing a previous wooden structure. There were two baileys surrounded by the Stour to the south, the Chilton Stream to the east and north, and moats. Still standing is the wall that once separated the two baileys, next to Ladies Walk, over half of the outer bailey, and a few sections of flint wall of the southern side of the inner bailey. Documentary evidence reveals the names of stone towers that once protected the inner bailey–Auditor’s Tower, Oxford’s Tower, Maiden’s Tower and Constable’s Tower. Elizabeth de Clare, possibly the family member who lived there for the longest continuous period, seems to have built an ornamental garden.
The portion of the flint keep on top of the motte and the wall at Ladies Walk were recently restored by English Heritage; this somewhat altered the profile of the keep. A flagpole was also added and at most times a flag with the de Clare family arms is flown. (Three red chevrons on a yellow background).