The castle’s story stretches back to the medieval period when it was associated with Walter de Ridelesford. By the 16th century, historical records describe the structure as being in ruins, though this didn’t prevent it from being inhabited during the 17th century by Piers Archbold, son of Richard Archbold of Kilmacud. The Archbold family’s occupation marked the beginning of the castle’s transformation from a defensive structure into a more comfortable dwelling.

The tower house itself is a substantial three-storey structure with a rectangular ground plan measuring 8.7 metres in length and 5.6 metres in width internally. Angular towers project from the northeast and southwest corners, featuring the narrow slit openings typical of defensive architecture. A spiral staircase winds through the southwest tower, which also houses a modern entrance. The building has undergone significant modifications over the centuries; its exterior is now covered in pebble-dash, topped with a slated, hipped roof, whilst the southwest tower sports a more recent cone-shaped addition. Inside, later inhabitants have made their mark with inserted two-light windows on the main floors and a fireplace built into the eastern wall.

Today, Knocklyon Castle stands as a fascinating example of how Ireland’s medieval fortifications evolved over time, adapting to changing needs and tastes. From its origins as a defensive stronghold to its conversion into a residence, and through periods of ruin and renewal, the castle embodies centuries of Irish history in its modified walls and repurposed spaces.