This rectangular tower, measuring approximately 10.65 metres east to west, once formed part of a larger defensive complex that included a nearby motte and bailey castle and wayside cross. The 1657 Down Survey map reveals the tower in its prime, depicted as a multi-storey structure with a two-storey house attached to one side, complete with a gable chimney stack. During this period, the lands belonged to Richard Nugent, Earl of Westmeath, one of the prominent Anglo-Norman families who dominated the region.

Today, only fragments of the castle's former glory remain; the northwest wall stands two storeys high, whilst the northeast and southwest walls survive to roughly half their original height. Built from rough, undressed limestone blocks laid in irregular courses, the structure features rounded corners and battered walls at its base, typical defensive features of Irish tower houses. Inside, evidence of the castle's domestic arrangements can still be traced: springers of a stone vault at first-floor level, the remnants of a chimney flue in the southwest corner, and a narrow mural staircase, now blocked, in the northwest corner. Small window openings punctuate the thick walls, which measure 1.25 metres across.

The castle's name derives from a mill that once operated nearby, though by 1837 the Ordnance Survey Letters noted that not even its site could be identified. However, traces of the medieval landscape persist in the surrounding fields, where old banks and earthworks form irregular enclosures to the east and southeast of the tower. An ancient roadway runs along the western side of the castle, crossing what appears to be a dried-up millrace via an old bridge; this water channel, clearly visible in aerial photographs from 2011, was diverted from the River Glore through a cut made 100 metres northeast of the castle. These remnants, along with a mysterious square tower noted 90 metres away and marked simply as 'Tower' in gothic script on the 1837 Ordnance Survey map, hint at a once-thriving medieval settlement that commanded this strategic position above the river valley.