What makes the place quietly odd is the entrance: a freestanding stone ramp leads up to a first-floor doorway, but the ramp stops short, leaving a gap of roughly two metres that would once have been crossed by a timber drawbridge, now long gone. The building was never a castle. It was a Napoleonic-era barracks, built around 1810 to support a coastal signalling network, and it came with gun loops, enfilading walls, and a mechanism for pulling up the gangway at night.

Between 1804 and 1806, the British Admiralty constructed a chain of signal towers around the Irish coastline to watch for French naval activity. Where line-of-sight between towers proved difficult, a small number of more substantial enclosed barracks were added to the system, and Kerry received four of these, all built to a strikingly similar plan. The Bolus example, constructed of random rubble sandstone with dressed quoins at the corners, is a three-bay building, originally two storeys or a single storey over a raised basement, with small annexes attached at each gable end. It probably also carried a signal mast. The compound enclosing it measures roughly 26.5 metres by 24 metres, with square bastions at each corner fitted with splayed gun loops designed to provide overlapping fields of fire along the outer walls. Whether the enclosed barracks were built to house larger crews than a standard signal tower, or simply to house crews of the same size in rather less miserable conditions, remains uncertain. The signal towers of the south-west coast were decommissioned around 1815, and the barracks at Bolus was presumably abandoned at the same time. The site sits about 570 metres from a separate signal tower at Bolus Point, with views stretching across Ballinskelligs Bay to the south and St. Finian's Bay to the north. A little over 30 metres to the west, a well-preserved Second World War lookout post adds another layer to the ridge's long history of coastal watching, while roughly 1.8 kilometres to the east lie the remains of an early medieval ecclesiastical enclosure at Cill Rialaigh.