Dundrum Castle was built by the Norman knight John de Courcy around the start of the 13th century, after his invasion of Ulster, on a rocky hill above the village of Dundrum. King John captured it in 1210, and Hugh de Lacy then added the massive round keep. It was dismantled under Cromwell and is now a State Care monument.
The ruin is substantial, and the views are the reason to climb up: south over Dundrum Bay to the Mournes, west to Slieve Croob and east across the plains of Lecale. It is free to explore and about 6 kilometres north-east of Newcastle.