If you make the trip to Northern Ireland and want to see a castle then I'd put Dunluce at the top of your travel list. It's not exactly a hidden gem; most people know it and would recommend it, but it's definitely somewhere that lives up to its reputation. Crumbling atop grassy cliffs over the ocean it looks straight out of an Irish legend; a fitting look given the castle's storied history. Legend has Maeve Roe McQuillen fell in love with Reginald O'Cahan, but her father didn't approve of the match and held his daughter captive in the north eastern tower of Dunluce. However as Maeve pined for her true love, until one night a servant helped her escape to the Mermaid's Cavern underneath the castle where Reginald was waiting for her. However it was a stormy night and their boat was dashed against the treacherous coastline. Maeve's body was never found and her ghost still haunts the castle to this day. You can still explore the Mermaid's cave underneath the castle and hear the crashing waves echo.
P.S. It's not all rough terrain to see this castle; we went off the paved paths and stairway for these pictures.