Main Mill Building at Museum of the Welsh Woollen Industry.

The Main Mill Building at the Museum of the Welsh Woollen Industry is an early 20th-century woollen mill, consisting of two parallel blocks that are aligned roughly northwest to southeast. The southern block is L-shaped, and the space between the blocks is currently covered with a roof. The building is two storeys high with a loft, constructed from rubble stone, featuring pitched roofs covered in slate and cast iron rainwater goods.

The large windows, primarily 16-pane with swivelling upper lights, are generally adorned with red brick dressings and arched heads. The elevation facing the yard has 16 bays, with the first floor's ninth bay containing a boarded timber loading door topped with a gabled attic, and a similar loading door above it. On the ground floor, the second, fifth, and eleventh bays have doorways. Beneath the fifteenth window, there is a cast iron overshot waterwheel. The left gable end has a later brick lean-to and an attic loading door.

To the left, there is a two-storey block with a monopitched upper storey, positioned at an angle to the parallel northern range, which is also built of brick. The northern block has doors on the left and windows on each floor to the right. Its gable end features three first-floor windows, two smaller loft windows, and two ground floor windows in the outer bays. The rear elevation consists of 12 bays, with similar glazing to the southern front. The eastern gable has a 12-pane window in the attic and loading doors on each floor, positioned to the left. The southeast return of the southern block has seven square 12-pane windows on the first floor, which appear to be reduced in depth, with red brick dressings. The ground floor has similar 12-pane windows with yellow brick dressings, and slate sills throughout. There is a blocked former window located in the fourth bay from the left.

The buildings retain early 20th-century roof structures in the lofts, along with wooden floors and beams that have been reinforced in places with iron stanchions. The ground floor features quarry tiles, and power shafts remain in situ. The machinery includes a mix of units from the working mill, reportedly imported in the 1950s, such as a mule with 400 spindles located in the first floor rear block, as well as museum exhibits that illustrate the history of the woollen industry.