It was King John's father, Henry II, who first had ideas of colonising Ireland, in 1171. He built a temporary fortress here, but it was not until 1204 that an impregnable fortress was built. The base of King John's tower was discovered in 1986. The walls were metres thick, fashioned from limestone quarried from the Wicklow Mountains. When King John built the castle, boats were able to sail across the Irish Sea, all the way up the Liffey and into the moat.

The castle was, for nearly eight centuries, the home of a succession of viceroys who governed Ireland in place of the English king. They lived there in elegant state apartments until 1684 when a huge explosion caused a terrible fire. The old castle was so badly damaged that it had to be demolished, except for one tower, which still stands. The castle was rebuilt in the 1700s and the state apartments were furnished in high Georgian style.