The square tower house, measuring roughly 9 metres on each side, appears on the 1656-8 Down Survey barony map as a roofless gabled structure with a central tower. Historical records from the Civil Survey of 1654-6 reveal that by 1640, the site comprised not just the castle but also a stone house and a decayed chapel, all sitting within 184 acres owned by the Lord of Slane, who controlled almost the entire parish.

The castle’s defensive architecture reflects the turbulent times in which it was built. A small projecting tower, measuring 3.5 metres square, juts from the western corner of the northwest wall, housing essential facilities including a garderobe with four narrow slit windows; two of these cleverly breach the corners to provide additional defensive coverage. The ground floor features a substantial barrel-vaulted chamber, approximately 6.8 by 6 metres, with its original southwest doorway now blocked and later doorways and windows inserted into the northeast wall. The projecting tower contains narrow defensive lights and a double-splay window on its northeast wall, along with the remains of a garderobe chute.

What makes Newstone Castle particularly intriguing is its vertical circulation system, though much of it is now inaccessible. A newel staircase, entered through a doorway in the southwest wall of the tower, once rose above the garderobe chamber, though it’s now blocked. The tower itself retains its own barrel vault running northeast to southwest above the garderobe chamber, while the main structure’s ground floor vault runs perpendicular to this. The castle stands adjacent to a later house structure, creating a complex of buildings that tell the story of this site’s evolution from medieval stronghold to more peaceful domestic use.