“And he sent a fleet of ships to Anglesey, and they gained possession of Arfon. And then was made the bridge over the Menai; but the bridge broke and countless numbers of the English were drowned and others slain.” –Brut y Twysogion, Peniarth Manuscript 20 (Chronicle of the Princes).
On November 6th, 1282, the Welsh achieved an historic victory at the Battle of Moel y Don. The English had thought to surprise them by crossing the Menai Strait and driving down the coast to Aber (Garth Ceylyn), Prince Llywelyn’s seat on the Welsh north coast.
The Menai Strait is the narrow body of water that separates Anglesey from Gwynedd proper. The river-like flow changes course according to the tide. The rising tide approaches from the south-west, causing the water in the Strait to flow north-eastwards as the level rises. It then flows counter-clockwise around Anglesey until, a few hours later, it shifts, and begins to flow the opposite way.
At that point, the water runs through the Strait in a south-westerly direction from Bangor (on the mainland) and Llanfaes (on Anglesey). It was Llanfaes where the English commander, Tany, held his troops, waiting to cross to attack.
By the time the tide reverses course, the tidal flow from the Caernarfon end has weakened, even if the tide continues to rise in height throughout the straight. Thus, slack water between Anglesey and Gwynedd tends to occur approximately one hour before high tide or low tide.
On the day of the attack, the English hoped to cross near high tide, when the water would be it’s calmest. They began at noon, with high tide at 1 pm. But the Welsh swept down from the heights above the beach and stopped them. The ferocity of their attack forced the English soldiers back across the bridge, which then broke under the weight of the men, horses, and equipment. By then, the tide was in full spate, moving west at 2.5 knots.
History records that 16 English knights, another 16 squires, and 300 footmen died that day.
Prince Llywelyn believed he could capitalize on this victory by leaving his brother, Dafydd, in charge of Gwynedd and going southeast to Powys to garner support among the other Welsh lords of Wales. Unfortunately, he was lured into a trap at Cilmeri and killed only a month later, on 11 December 1282.