This castle dates back to 1204 and currently houses Ireland's State Apartments, the Chapel Royal, the Undercroft, Police Museum, and Charles Beatty Library. It was the seat of British rule in Ireland until 1922, and is now used for hosting official state visits. You can walk the grounds for free, but there isn't much to see without a guided tour. The guided tours are informative, but, at one hour, can be lengthy if you are not particularly interested in Irish history. A highlight for many is going underground to see parts of the old city wall and moat.