The guardianship monument at Mousa comprises the remains of an Iron Age broch, situated on a low promontory on the W shore of the uninhabited island of Mousa. The broch is surrounded by a low stone bank. It is recorded that beehive huts once existed within the enclosure and traces of small, robbed structures are still visible near the entrance to the broch.
The monument was first scheduled in 1882, but the file for this cannot be traced. It was taken into care in 1885, but this area is not precisely defined and is certainly inadequate area to protect all of the archaeological remains: the present scheduling rectifies this.
The broch at Mousa comprises a drystone tower, about 15m in diameter preserved to a height of 13.3m. Its profile is not straight but swells out at the base and tapers towards the top (12m diameter); this curvature may have been accentuated by settlement. Entered from the ground floor on the W, the narrow passage leads into a subcircular interior (6.1m diameter). The upper levels of the doorway are now modern; a later doorway (pre-1851) had been built above the original, but was removed in 1919. The interior is cluttered with several phases of later prehistoric subdivisions, including a wheelhouse, but a water tank appears to belong to the earliest phase. There are two scarcement levels, a doorway opening into the courtyard from each. The solid base of the broch contains three intramural cells, but from a height of 3m the wall is hollow, containing six superimposed galleries, floored and roofed with large slabs, and with vertical apertures and recesses in the inner face. A stair of narrow stone steps rises through these galleries to the wall-head, part of which is reconstructed. This retains part of its original capping, a slanted slabbed roof.
Norse use of the broch is recorded in the sagas: in AD 900 Moseyarborg was used a shelter by an eloping couple; and in AD 1153 it was used as a refuge during a family dispute.
The area to be scheduled is irregular on plan, with maximum dimensions of 60m from N-S by 75m from E-W, to include the broch, surrounding outbuildings and enclosing bank and an area around in which evidence relating to their construction and use may survive, as marked in red on the accompanying map extract. All modern above-ground structures, such as information panels, are excluded from the scheduling.