Castle-upon-Alun House including attached stone walls enclosing garden, and outbuilding

Castle-upon-Alun House is a long two-storey range with an attached rear wing, dating from the 18th century and later. The house sits on a long garden front facing south. The exterior is rough cast over masonry, topped by steeply pitched slate roofs. The main range has two end stacks made of masonry and two ridge stacks; the rear range has three masonry stacks. Windows are generally 12-pane horned sash windows with flat heads and stone sills.

The main range is divided into three sections. The central section is lower and has two windows. It is flanked by a symmetrical three-window addition to the left and a four-window addition to the right, with two gablets. The central section has two narrow sash windows on the upper story, offset to the left. The lower story has a 20th-century half-lit door to the right and a four-light 20th-century window to the left. The left addition has three sash windows on the upper story and a central half-lit door with overlight, flanked by sash windows on the lower story. The right addition has two gablets with wide bargeboards, each containing two half-dormer sash windows. Below the gablets are three symmetrically placed windows.

The rear wing is set to the west. The west gable end of the main range has no openings. The west side of the rear wing has five windows on the upper story and three on the lower, with two gabled attic dormers featuring multi-pane windows. The north gable end of the wing has wide bargeboards, and on the ground floor are two sash windows and a small 20th-century window to the left.

Attached rubble garden walls define the garden to the south and east of the house. A rubble wall doorway near the southwest angle likely incorporates medieval stonework, with a similar Gothic doorway at the southeast angle of the garden walls. A square stone outbuilding is incorporated on the east side of the garden.

The entrance hall features a large stone fireplace with a moulded Tudor arch on the south side, and a straight staircase on the west side. The ceiling is beamed and pargetted. The reception rooms are to the south of the hall, with a main room at the end. A boarded door leads to a fireplace staircase to the left of the Victorian fireplace; the doorway is set at a 45-degree angle. A small lobby is to the right of the fireplace, possibly indicating the original entrance to the house, and now leads to a door to the garden. Interior details include wainscot panelling, decorated and moulded coving, and plastered ceiling beams.