Dating back centuries, this fortification appears as a castle on the Down Survey maps of 1655-6, marking its significance in the local landscape. The three-storey structure features a square tower projecting northward from its northeast corner, built from coursed limestone with roughly dressed quoins, though much of the stonework now lies hidden beneath later plaster.

The castle’s defensive features remain remarkably intact. Entry is through a round-headed doorway in the north wall, where visitors pass beneath a murder hole in the entrance lobby; a sobering reminder of the tower’s military purpose. The vaulted ground floor measures 7.08 metres long by 5.2 metres wide, with access to a stair turret through another round-headed doorway. The second floor, reached via a two-centred arched doorway, contains a garderobe chute in the southeast corner, its weight supported by corbels. This small circular chamber, lit by a single opening and accessed through a narrow round-headed door with hammer-dressed jambs, speaks to the practical considerations of medieval life. A 1773 drawing by Beranger shows the tower once featured stepped crenellations at parapet level, though these have since been lost.

Archaeological work in 1997 revealed the castle’s broader context within the medieval landscape. Excavations uncovered a curving ditch running northeast to southwest, measuring 30 metres long and up to 2.4 metres wide. The ditch fills contained charcoal, mortar, flint and animal bones, whilst finds including a decorated bone comb, stick-pin and knife suggest activity from the 12th to 13th centuries. A stone causeway crossed this ditch, and the evidence points to extensive early medieval and post-medieval activity in the area, with the ditches likely serving as medieval field boundaries. Today, while modern development encroaches with business parks and access roads, Grange Castle endures as a testament to centuries of Irish fortification and settlement.