The building is unusual not simply because of its age, but because of the layering involved: a later masonry structure grafted onto an earlier earthwork, with one of its chambers actually built into the body of the mound itself. What remains today is a rectangular structure roughly 14 metres long and 6 metres wide, reduced now to a single storey, its interior divided into two chambers by a wall inserted at some point after the original construction. Each chamber retains a lintelled doorway and a splayed rectangular window set into the south-west wall, and there is a narrow opening in an embrasure in the north-west wall. The south-east chamber has a small vaulted alcove tucked into the north-east wall, where the motte's earthen mass presses close.

By the mid-seventeenth century, the building was already a ruin. The Civil Survey of 1654 records that 332 acres at Culmullin were in the ownership of the Lord of Slane, and notes the presence of a stone house and a church on the land, both described as ruined at that time. Just a few years later, the Down Survey barony map of Deece, drawn up in 1658, depicts what appears to have been a substantial two-storey house, probably with a return, standing close to the church at Culmullin. Whether the surviving vaulted structure and the house shown on the Down Survey represent the same building in different stages of its life, or are related but distinct elements of a larger complex, the physical evidence that remains speaks to a site that was already long past its working life before the Cromwellian surveyors came to map it.