Groes Onnen Windmill Holywell · Flintshire · Wales

Groes Onnen Windmill stands on Pen-y-Maes Road at Holywell, set back from a bend in the road on the edge of the town centre. The site has a longer milling association: Basingwerk Abbey is believed to have erected a windmill here, although the present surviving tower belongs to a later phase of windmill construction. The existing structure is probably late eighteenth century or about 1800.

The surviving mill is a conical stone-built tower windmill with a plinth. It stands to four storeys and has camber-arched openings with modern small-pane, top-hung windows arranged in regular tiers. The north side has two modern doors, the upper one opening onto a first-floor balcony carried on raking timber struts rising from the plinth. The cap has been replaced with a weatherboarded boat-shaped cap and timber balcony, except on the former sail side. The tower had been restored over the decade before the 1990 description and had recently been whitewashed in autumn 1990.

The mill is one of the few Welsh windmills to survive to full height. Later accounts describe it as a restored residential property, without sails, and as a visible local landmark on the approach into Holywell. It was listed at Grade II in 1951 for its importance as a prominent landmark and as a rare surviving Welsh example of a full-height windmill.