Built by Sir William Stewart after he received a grant of 1,000 acres in 1610, this defensive structure was completed by 1611 as a formidable bawn; a fortified enclosure typical of the Ulster Plantation period. Contemporary records describe it as being constructed of lime and stone with two flankers, one containing a room suitable for storing munitions or housing prisoners, topped by a guard room and an open fighting platform with a sentinel house at its highest point. By 1619, Stewart had added a thatched house within the protective walls, which housed a Scottish minister and his family, reflecting the plantation’s dual purpose of defence and settlement.

The surviving ruins reveal the ingenuity of early 17th-century military architecture, with two distinct defensive towers still visible amongst fragments of the original bawn walls. The southern flanker is a circular two-storey tower rising about 4.5 metres, its base reinforced with a battered foundation that drops sharply to the lough shore below. Gun loops on each floor provided covering fire for the eastern and southern walls, whilst a window at first-floor level and evidence of timber joists speak to its dual military and residential functions. The northern flanker takes the form of a salient-angled bastion standing 5.5 metres high, with its ground floor accessible only from above; likely the prison or munition store mentioned in the 1611 description. Both structures retain evidence of their original timber doors and were crowned with defensive parapets, whilst the remaining section of western wall preserves the chase for a timber jamb and socket for a substantial draw-bar, marking the location of a former gate.

Archaeological investigations carried out in 1998 and 2005 found little evidence of earlier occupation, though two ditches discovered during monitoring work were dated to the late 18th or 19th century, suggesting the site continued to see activity long after its military significance had waned. Today, Fort Stewart stands as one of Donegal’s more evocative plantation ruins, its strategic position on Lough Swilly and substantial remains offering visitors a tangible connection to the complex history of the Ulster Plantation, when English and Scottish settlers transformed the landscape of Ireland’s northern counties under the watchful protection of fortifications like this one.