King John's Castle dominates the King's Island bank of the Shannon, a five-towered Norman fortress in dark stone, with thick curtain walls and a deep dry moat now grassed over. The visitor centre, opened after a 2013 refit, takes you through 800 years of siege, famine, and politics on touch-screen displays that are unexpectedly good — particularly the section on the 1690 siege, where you can stand on the original earthworks built by the Williamites and look out at the breach they made.
The towers themselves are open to climb, and the views from the parapets over the river and the medieval streets of King's Island are the best in the city. Allow 90 minutes for the full visit. Combined tickets with the Hunt Museum or the medieval walking tour can save money for anyone planning to do all three.