The most impressive and best preserved part of Conisbrough Castle is the circular keep, which has six wedge-shaped buttresses placed equidistantly around its circumference. The keep was built by Hamelin Plantagenet, illegitimate half-brother of Henry II, sometime around 1180. The keep, and a circuit of curtain walls that he added soon after, were built on the site of a castle founded at the end of the 11th century by William de Warenne, the first Earl Warenne. Hamelin inherited the title and estates through his marriage to Isabel, daughter of the third Earl Warenne. The design of the keep is unique in this country; the only other similar example was built on Warenne owned land in France and might also have been the work of Hamelin Plantagenet. The keep, built of fine limestone ashlar, stands close to its original height of 30.5m (100ft) tall. It is dark and gloomy inside due to an almost complete lack of windows, and its circular shape means it does not provide very spacious accommodation. The owners of the castle would undoubtedly have enjoyed the more pleasant accommodation provided by buildings within the bailey.

When the last Earl Warenne died heirless in 1347, the castle reverted to the crown. By 1538 the castle was already in a state of ruin. The castle, made famous by Sir Walter Scott's book, Ivanhoe, is now managed by the Ivanhoe Trust on behalf of English Heritage.