This tower house, constructed from roughly coursed limestone rubble, dates back to at least the early 17th century when it appeared on a contemporary map of Moydow barony. The structure measures 11.4 metres northeast to southwest and 9.3 metres northwest to southeast, with impressively thick walls spanning 2.3 metres. Its massive base batter, a defensive architectural feature that slopes outward at the foundation, would have made the castle particularly difficult to undermine during sieges.

By 1636, the castle had already fallen into ruin, as recorded in the inquisition post mortem of William McBryen Murtagh of Dowclone (now Mosstown), which listed "one ruinous castle in the townland of Ballyknock". Despite its deteriorated state, evidence of the tower house's domestic arrangements remains visible; fireplaces can still be seen in the southeast and southwest walls at ground floor level, whilst the probable location of the main doorway in the now collapsed northeast wall can be inferred. The structure originally stood at least three storeys high, though time and the elements have taken their toll on the upper levels.

What makes Ballyknock particularly intriguing is its defensive layout: the tower house sits centrally within a circular bawn, a fortified enclosure typical of Irish castle design. This bawn appears to have been cleverly constructed atop an even earlier ringfort, suggesting that this hillock has been a strategic defensive position for centuries, with successive generations recognising its military value and building upon the fortifications of their predecessors.