Kisimul Castle stands on a rocky islet in Castlebay, reached only across the water. It became the principal seat of the MacNeils of Barra, whose authority was rooted in the maritime world of the Outer Hebrides. The precise date of the surviving buildings is debated, but much of the enclosure is generally associated with the later Middle Ages. A curtain wall, tower house, hall, chapel, and service spaces crowded onto the small rock, creating a compact residence protected more by sea and seamanship than by extensive land defenses.

From Kisimul the MacNeil chiefs supervised island lands and sea routes. Gaelic tradition celebrated their independence and naval skill, while hostile accounts sometimes portrayed them as raiders or pirates. Both images reflect a society in which warfare, cattle, fishing, tribute, and control of boats were closely connected. The castle's sheltered landing and limited storage required careful management. It was a household and political center as well as a refuge, its small rooms serving chiefs, family members, retainers, clergy, and workers who depended on mainland and island communities for supplies.

Changing law, debt, and estate economics weakened the old clan order. The MacNeil chief sold Barra in 1838, and Kisimul ceased to be a functioning residence. Roofs collapsed, and stone from the castle was reportedly reused in local building and fishing works. Even as the fabric declined, the island fortress remained central to Barra identity and to the wider memory of Clan MacNeil. Nineteenth-century visitors encountered a romantic ruin, but its deterioration also recorded the displacement and economic pressures experienced across the Hebrides.

Robert Lister Macneil, an American-born descendant who became clan chief, purchased the Barra estate in 1937 and began reclaiming Kisimul. Twentieth-century reconstruction restored roofs, floors, and selected interiors, using surviving evidence alongside contemporary judgments about how a medieval Hebridean castle should appear. The work saved the enclosure but also means that present fabric must be read carefully: medieval masonry, later collapse, and modern rebuilding coexist. Kisimul again became a ceremonial focus for the clan, though not a continuously preserved medieval household.

In 2001 the chief leased Kisimul to Historic Scotland for a symbolic annual rent of one pound and a bottle of whisky. Public stewardship brought new surveys, repairs, and debate about conserving a marine monument exposed to salt, storms, and difficult access. Periods of closure have underscored how demanding that environment remains. Kisimul Castle's significance lies in the union of place and community: a compact fortress shaped by island lordship, abandoned after economic change, and deliberately recovered as an emblem of Barra and MacNeil identity.