In design Barra Castle is an intricate and singular variation of the L plan type, the main block lying north/south with a circular tower at the south west. At the south east a D plan tower . An 18th century addition runs eastward from the north end of the main block forming a square court closed on the east side by a wall containing the entrance. The basement of the main block is vaulted. In its present form it is chiefly the work of George Seton who was granted the estates in 1598.

The plan is a somewhat complicated variation of the L, with the main block lying north and south and a wing projecting eastwards at the south end so as to form two entrants; in the other re-entrant is a circular stair-tower , corbelled out to the square at the top to form the usual watch-chamber; and there are two more round towers, with conical roofs, projecting at the south-west corner of the wing.  The 18^th century addition extends eastwards at the north end of the main block, thus forming three sides of a square, a curtain wall joining this and the wing to enclose a forecourt.  The walls, of coursed rubble, are three storeys and a garret in height.  The dates 1614 and 1618 appear on the upper part of the building, along with the symbols of George Seton, the intertwined three crescents.

The entrance is in the main re-entrant, within the courtyard, and at eaves level above is a crow-stepped gablet – an unusual feature.   The basement is vaulted and contains the customary kitchen and cellars.  The turnpike stairway is unusually wide.  The Hall on the first floor has been a fine apartment, subdivided at an early date to form dining and drawing-rooms, pine paneled, with a tiny chamber off, in the south-west tower.  There is a bedroom at this level in the wing, also with a small chamber in the south-east tower.  There is ample bedroom accommodation above and in the extension.