The 1st edition OS map of 1888 records Melin-marchog, with two small buildings, one of which is adjacent to a pond on the Afon Marchogion. The same buildings are unnamed on the 2nd edition (1900), and had disappeared by the time of the 3rd edition, 1920.
In November 2008 the site of the mill was barely distinguishable, a small flat plot with fragments of stonework. The modern footpath buries the south-east corner of the plot. Above the mill site the path passes through a rock-cut defile, which may be the former leat; there are exposures of the conglomerate beds which yielded millstones in the Benllech area. The part-collapsed dam of the upper pool, formed of very large irregular slabs, is at SH5150182237 and part of the lower walls of the mill cottage are at SH5154782272.
Melin Castellbwlchgwyn is named in the 1352 Extent of Anglesey; the name Marchog, Knight's Mill, was adopted when the Bulkeley family acquired the mill some time before 1585. In 1827 it was recorded as being in "a bad state of repair and very little water". It went out of use c.1868.
W J Crompton, RCAHMW, 19 November 2008.