Caer Eithin Engine House, Berthlwyd Colliery

Caer Eithin Engine House, located at Berthlwyd Colliery, is a complex structure built from local sandstone rubble with red brick features. The main section measures approximately 10 meters by 8 meters and stands about 5 meters high. A large brick-vaulted tunnel runs through the building from north to south, widening at the north face. From this tunnel, a small sloping tunnel extends downwards to the east.

To the south, there are two substantial wings made of brick and stone, with an area of raised ground between them that contains two small parallel blind tunnels in brickwork. There were additional buildings to the south and east that are no longer present, as well as a top storey. Historical evidence suggests that a steam engine operated at ground level, likely with its steam-raising equipment located on the south side, possibly on the raised platform. It is believed that a small amount of coal was drawn up from the mine to fuel the boilers. The south face of the structure shows signs of an abutting roof, which is thought to belong to a boiler house, although no chimney position is visible.

The top storey has been reinforced with steelwork to create a larger platform, with five rolled steel joists cantilevered outwards on the east and west sides, forming a set of tied brick arches between them. The engine installed at this level is believed to have been electrically powered. Local oral tradition indicates that there was a ladder on the west face for access to the upper engine house.

Additionally, there is another platform made of steel joists with brick arches, located about 1 meter lower, which serves as a roof between the south-pointing wing walls. On the east face of the eastern wing wall, there are marks of another roof, suggesting there was an abutting building on this side, with a narrow opening providing pedestrian access between the surviving structure and this lost building.