A 2-storey gaol, with main E-W range incorporating the N entrance to Bunkers Hill, and S wing extended to the perimeter wall in 1867, and a lower wing on the E side, giving an F-shaped plan. The elevations are mainly of coursed rubble-stone walls with freestone dressings and monolithic surrounds, with hipped graded-slate roof on wide eaves, and with freestone stacks. The severe 3-bay entrance, between the splayed abutting walls of the outer perimeter, is of coursed, rock-faced limestone. It has a central tripartite doorway of minimal classical distinction, and with '1829' incised into the entablature. It has recessed small-pane flanking windows and segmental-headed central studded door. Windows are 16-pane hornless sashes.
Further L, behind the perimeter wall and opening to one of several prison yards, is a 7-window elevation, in which the upper storey has iron-barred cell windows with recessed small-pane casements. In the lower storey, at the R end, is a studded door with small-pane sash window to the R and blocked window to the L, opening to a small kitchen yard. On the L side is a courtyard wall comprising a freestone coped dwarf wall and tall vertical slate slabs, and incorporating a freestone doorway surround. To the L of the kitchen yard is a studded door with barred window to the R and 2 barred windows to the L. Iron railings, with gate, are built between the end of the wall and the perimeter wall.
In the E end wall is a panelled door with flanking lights and overlight. Above is a studded door, which led by means of a temporary walkway to the gallows in the perimeter wall, and small-pane overlight.
At the SE end of the main range is a single-storey punishment wing under a roof of small slates, which faces W toward one of the main exercise yards. It is open-fronted on 2 cast iron posts. At the N end are 6 stalls to a wooden treadwheel, which pumped water to the prison well, which is reached through a boarded door on the R. Further R is another boarded door, to a whipping room. At the R end is a monopitched ty bach with wooden seat, under an original roof of large slates, and wall continuing to the perimeter wall, closing off the yard but incorporating a doorway.
The S wall of the main range, on the E side of the S wing, has 7 upper-storey cell windows, of which the 4th and 5th are set lower and have 2-light casements, with a single-light and barred window further R. In the lower storey is a small-pane sash window to the L and a studded door flanked by 2-light small-pane casements. A splayed angle with the S wing has a pair of 16-pane hornless sashes in the lower storey (lighting the chapel), a 2-light window upper L and 12-pane hornless sash window upper R.
The S wing is 4-window in its original form, with wider 2-window extension. In the original portion is a doorway to the R with barred overlight, and three 2-light windows, of which the outer are barred. In the upper storey are 4 barred cell windows, except the centre-R which is blind. The S wing is surmounted by a tall rectangular water tower with projecting cap on a deep moulded corbel table. Either side of the water tower are small-pane dormer windows lighting the central lobby of the main range. The extension to the S wing has 2-light windows in the lower storey and shorter but similar barred windows in the upper storey. A lantern has 4 similar windows.
Returning to the main range, on the R (W) side of the main entrance, facing a yard on the N side, is a 7-window elevation similar to the E side, except that there are 3 doorways instead of 2, and iron railings across the yard to the perimeter wall. At the end of the wall is an added lean-to ty bach, and another gate with iron railings. The W end wall has a triple barred window in the lower storey, a barred window above, both lighting the corridors, and smaller cell windows R and L. The S elevation has 7 cell windows in the upper storey, of which the 2 at the L end are blocked, and in the lower storey a studded door L of centre, and to its R 2 barred windows and 2-light window at the R end. Across the entire front are 6 stone-walled bays used for breaking up rocks, into which a monopitched projection has been inserted at the L end.
The splay in the angle with the S wing has two 24-pane sash windows in the lower storey and similar but shorter 16-pane and 12-pane sash windows above. The S wing has 4 upper-storey cell windows, of which the centre-L is blocked, and, in the lower storey, a doorway with overlight to the L and 3 barred windows. The R-hand window has a vertical joint beneath its L jamb, suggesting that it was originally a doorway. The extension of the S wing has 2 barred windows in each storey, shorter to the upper storey, and lantern with 4 similar barred windows.