The architectural historians MacGibbon and Ross attributed Borve Castle to Amy mac Ruari, dating its construction to between 1344 and 1363. That window of dates matches her life: she inherited Garmoran in 1346 as the sole surviving MacRury heir, married John of Islay shortly afterward, and was set aside by him in the early 1350s. Borve Castle was the seat of her remaining power. From here she also funded the parish church of Saint Columba at Nunton, with the eastern wall of Teampul Chalumchille dating to her lifetime. The Macdonalds of Benbecula, descended from Amy's son Ranald, occupied the castle until the early 17th century, when the family seat moved to the more comfortable house at Nunton.

The tower measures 18.9 metres by 11.3 metres, and stood at least 9 metres high. Its walls average 2.75 metres thick, narrower at the upper storeys to save weight. The north wall has almost entirely collapsed, but the others remain. There were at least two timber floors above the basement, reached by an entrance in a projecting wing on the south side that opened into the first storey, the medieval security feature designed to make ground-level assault harder. The first floor would have housed the lord's hall. Above it, a chamber for private living. Below, dark storage. This was not a castle for sieges so much as a fortified residence: a sign of status, a refuge in trouble, a wind-break in everyday life.

Archaeological work in 2018 revealed something unexpected. The tower was not built all at once but in three distinct phases. The first walls were narrow, formal, with regular courses and sandstone quoins, the work of a competent mason following pattern-book conventions. Later, the eastern side was reinforced with a much thicker outer skin, large stones with intermittent coursing, no windows on that face. Later still, the western side was treated similarly but more roughly, without proper coursing, though pierced with many windows. Whoever did the work on the western side wanted light. Whoever did the east wanted protection. The castle's story is written in those wall sections, an accumulation of fear and confidence across the centuries it was lived in.

Borve Castle was given Scheduled Monument status in 1993, which protects the ruin in its current state. There is no roof to restore, no living quarters to reconstruct. What remains is a presence on the southwest coast of Benbecula near Lionacleit: a stone witness to medieval Hebridean power, to a woman who held a lordship in her own right when that was rare anywhere in Europe, and to the slow process by which fortified noble houses became farmhouses, then ruins, then preserved memories on the wind-bitten machair.