Measuring just 7 metres north to south and 5 metres east to west, this modest structure sits 90 metres south of a larger tower house, with the River Glore flowing 130 metres to the north. The tower’s stone footings are now largely overgrown, with the interior sitting lower than the surrounding ground level, possibly due to past excavation. The best-preserved section remains at the southern end, whilst considerable mounding at the northeast corner suggests structural collapse over the centuries.

When the Ordnance Survey visited in 1837, they recorded that Millcastle took its name from both a castle and a mill that once stood here, though even then the mill site could no longer be identified. The surveyors noted that the tower appeared to be of the same age and style as the nearby castle ruins, leading modern archaeologists to suggest it may have served as an angle tower for a bawn, or fortified enclosure, associated with the main tower house. The site’s defensive importance is evident from its proximity to a motte and bailey castle and a wayside cross, both located 290 metres to the north.

Archaeological evidence reveals a complex landscape of earthworks surrounding the tower, including a large rectangular enclosure to the northeast defended by an earthen bank and external fosse, best preserved on its western and northern sides. Between the tower and castle, further banks and enclosures are visible in aerial photographs, though many are irregular and poorly preserved. A particularly intriguing feature is a water channel that once ran east to west, diverted from the River Glore through a cut in the riverbank 100 metres north-northeast of the tower; this millrace, clearly visible in 2011 aerial photographs despite being dried up, provides the only remaining evidence of the mill that gave both the townland and castle their names.