Built before 1619 by Sir John Stewart on lands granted to the Duke of Lennox in 1610, this fortified house originally stood three storeys tall with four distinctive corner turrets perched on moulded corbels; a defensive feature borrowed from Scottish tower house design. Though the building was virtually complete with its roof intact as late as 1846, time has not been kind to Mongavlin. Today, only the turrets on the south gable survive, whilst the northwest corner has crumbled to barely 2 metres in height, and the interior lies choked with rubble and field clearance.

The castle’s construction reveals the practical concerns of its era: random rubble walls with hard sandstone dressings, rectangular windows fitted with iron bars and wooden frames, and gun loops flanking what was once the main entrance on the east wall. The south gable preserves the most complete section, including a large kitchen fireplace with an adjoining brick lined oven on the ground floor, and a smaller fireplace with roll moulded details on the second floor. The building measured approximately 12.85 metres by 5.3 metres internally, with wooden stairs likely connecting the floors, though no evidence of internal divisions remains.

A fascinating glimpse into the castle’s later history comes from a stone slab inscription, recorded around 1905 but now lost, which celebrated Elizabeth Hamilton’s purchase of the manor in 1704. The widow of Colonel James Hamilton, who died at sea in service to the crown, she annexed Mongavlin to the family’s estates across the water and was commemorated by her son, James, Earl of Abercorn and Viscount Strabane, for her prudent management and generous provision for her numerous offspring. The roof line of a single storey building and stone footings visible on the structure’s south face speak to later additions, recorded on 19th century Ordnance Survey maps, when the castle still served some practical purpose before its final abandonment to the elements.