This one sits in a pasture on a north-west-facing slope in Ballinafad, watching over the Curlew Mountains pass much as it did when it was new. What makes it quietly odd is the sheer density of its defensive thinking: gun loops angled to cover the entrance door, drawbar sockets on doors that could seal off each tower from the main block floor by floor, and what one scholar identified as a portcullis-like sliding gate above the first-floor doorway, though the physical evidence for that last feature is ambiguous. The lowest floor appears to have had no door or windows at all, suggesting it functioned as an unlit basement, accessible only from above. Box machicolations, small projecting openings through which defenders could drop objects on anyone directly below, survive on the outermost face of each tower just below roofline level, and two tall chimney stacks still rise from the east and north towers.

The building was put up in 1590 by Sir Richard Bingham, the Crown's governor of Connacht, to control a strategic chokepoint where the old route known as the Red Earl's Road threaded through the Curlew pass. In September of that year Bingham reported to the Privy Council the completion of a strong new fort in the strait of the Curlews, and this structure is almost certainly what he meant. A garrison of ten men under Captain John St Barbe held it, though not without difficulty: Red Hugh O'Donnell partially destroyed it in 1595, and it was sacked again during the 1641 rebellion. Crown forces reoccupied it in the mid-1650s as part of a wider network of fortifications underpinning the land settlements that followed after the restoration of Charles II, but by 1680 it had been abandoned. Four years later a visitor was already describing it as an ancient castle. The human traces around it are equally telling: in nearby Aghanagh graveyard there is a memorial dated 1659 to Winifred Hughes, wife of Henry Hughes who was governor of the castle, and the grave of Captain St Barbe himself lies in the same ground. The OPW carried out substantial repairs around 1940, rebuilding many of the embrasures and inserting concrete lintels, which means the ruin as it stands today is partly a twentieth-century reconstruction of a late sixteenth-century garrison post.