St Gwenfaen's Holy Well Rhoscolyn is one of the best preserved of the well chambers in Anglesey, but I would have to say one of the most difficult to find. There are many "holy" wells on the island, most with healing powers, but some were "cursing wells". The well at Rhoscolyn is said to cure mental disorders, and it was customary to throw white quartz pebbles into the water. (But I am afraid you would have to be mad to drink from the Well these days, 2008, as it is contaminated with dead sheep and sheep droppings).
The date of the stonework is unknown. It consists of a sunken antechamber with a stone paved floor and four seats across the corners. Beyond is an enclosed pool with seats at either side, and in front of that an open pool approached by three steps on each side. The water is held behind a vertical slab and spills over a notch into a narrow stone-lined channel and thence over the edge of the cliff. Yes, the well chamber is close to the edge of the cliff on the coast at Rhoscolyn.
I visited the site quite late on a day in February and due to poor directions I was fortunate to find the Well before the sun went down. But having said that I must say that I was also very pleased to spot another item which was just as interesting to me as St Gwenfaen's Well .... a grotesque finial  seemingly depicting the Devil holding a skull and sitting on a house that was alongside the lane that led to the Well. It's the best finial I have seen that wasn't atop a medieval church or cathedral !   

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Directions : [ Map of St Gwenfaen's Holy Well location ]

As I said the directions I had were pretty poor and I hope my directions are an improvement. From the Menai Bridge continue on the A55 toward Holyhead. Leave A55 at Junction 3 to Valley. Take B4545 to Trearddur Bay, then follow the sign to Rhoscolyn. Continue to St Gwenfaen's Church that sits on the hilltop. Parking is available close to the church. At the fork in the road there is a farm gate with a notice explaining the terms of use of the footpath. Follow the path south, passing buildings to either side of the path. Look closely at the first house on the left and you may see the Devil Finial atop the porch roof. Continue along the path toward the ladder style. Cross the style and continue toward the coast. Bear right heading north west between the lookout station and a stone seat on the opposite hill that looks like a Holy Well !
Persevere north for a few hundred yards down the bank. The Rhoscolyn Well Chamber is close to the cliff edge on the seaward side of the stone wall.