The central keep of Ireland's first and finest Norman fortress was built by John de Courcy soon after his 1177 invasion of Ulster. The massive walls of the outer ward were completed in 1242, while the red-brick gun ports were added in the 16th century. Inside, the keep is set up to recreate the days of de Courcy, and the site is dotted with life-size figures illustrating the castle's history. Guided tours run roughly every hour.
The castle overlooks the harbour where William of Orange landed on 14 June 1690, on his way to the Battle of the Boyne. A blue plaque on the old harbour wall marks the site where he stepped ashore, and a bronze statue of 'King Billy' himself stands on the shore nearby.