The Norman Keep was built in the early 12th Century by Robert Consul, Earl of Gloucester, replacing the timber defenses of Robert Fitzhamon, Norman Lord of Glamorgan. It was the prison for eight years of Duke Robert of Normandy, the eldest son of William the Conqueror, until his death in 1134 at the age of 80.
Additions made in the 13th and 14th Century included a gatehouse with a stone staircase leading to the stone bridge which replaced the timber drawbridge across the moat. The gatehouse was removed during landscaping in the 18th Century.(wiki)
Visiting with kids can be a challenge as the stairs to the top of the keep are really steep. Extra attention is needed. The top offers you great views of the castle's courtyard. Not busy at all this time of year.