Castle Mill, Mill Street, Ruthin, is a structure which is built of grey and red stone, and in its lower story is of the end of the 13th century, the same period as the adjacent castle; the upper stories are later. The mill wheel is in the middle of the north front, doubtless its original position, but the archway overhead has been restored probably several times. Within are two broad Early English doorways, and two slits for light that may be of any period. In the eastern gable is a closed Early English slit, and above it a cross of red sandstone. The mill-pond was in the Castle precincts, which makes it fairly certain that it was the lord's mill, and not the prior's. It is at present untenanted. Visited, 5th October, 1911.

(RCAHMW Denbighshire Inventory account)

Medieval building, much altered. Originally of one storey, walls of squared coursed limestone rubble, steeply pitched roof. The west gable has a tall blocked trefoil-headed lancet window, the dressings of red sandstone. In the north wall is a chamfered slit, with red sandstone dressings, blocked; another slit is damaged by an inserted window. There is a wide three-centred archway, its sandstone dressings partly renewed with limestone. In the east gable is a blocked lancet, with a cross patee standing on two steps outlined in red sandstone in the apex of the old gable. Subsequently (?1704) the building was raised to two and a half storeys. The walls being raised in red brick; a new slate roof was put on c. 1860, replacing six gabled dormers above six arch-headed first floor windows (visible on old engraving of the mill). Formerly there was a very wide internal backshot wheel, which was removed c. 1947; before [WW2] the mill had operated as a saw-mill. The interior has been completely altered for use as a builders' merchant shop; a brick addition at one end has a datestone inscribed: S . R : R / M / 1704. The second-phase trusses of the main building remain intact.

(A.J.Parkinson, RCAHMW, field notes 04.06.1979)

On the inner face of the front wall, set into a patch of brickwork close to a modern doorway, is a worn carving on red sandstone: the entire panel is made up of two blocks, apparently fitting together.
The upper block has a recessed rectangular panel containing a low-relief carving of a horse; the head is damaged. Above the carving are incised the letters 'I R', in probably 17th century style lettering: the 'I' has forked serifs.
Below the panel the bottom of the upper stone is arched to form the coved head of a second panel; in the curved soffit is a very worn but identifiable winged cherub's head. The lower stone makes up the rest of the lower panel, also rectangular and recessed: it contains a high-relief carving of the head of a man on a moulded pedestal. To either side of the pedestal the lower edge of the panel is chamfered. The head is badly worn, with almost no nose; it appears to be bearded. The hair is straight, possibly originally covering the ears, with a straight lower edge between ears and collar and curled forwards.
The date is not certain, but the style of the lettering is probably 17th century; the hair-style of the figure is unlikely to be post-Restoration, so the period 1620-60 seems the most likely. The original location and purpose is quite unknown: the initials 'IR' might give some clue, if they could be identified. The use of the cherub's head might suggest that it was some kind of memorial, but the significance of the horse is unknown.
Score-marks on the walls of the wheel-pit in the middle of the building suggest that the wheel was approximately 5.2m diameter by 3.9m breast.

(A.J.Parkinson, RCAHMW, field notes 14.6.1988)
B.A.Malaws, RCAHMW, 11 August 2005.

The mill has been converted to housing. The carved stone panel has been set in an exterior wall to the west of the mill where exposure to the weather is causing continuing deterioration: the features referred to in 1988 are almost unrecognisable.

B.A.Malaws, RCAHMW, 11 August 2005.