The Menai Strait is the narrow body of water, approximately 16 miles long, between mainland Wales and the island of Anglesey, called Ynys Mon in Welsh. At its center point, the Strait is roughly 1600 feet from shore to shore, widening to over 3000 feet at either end of the Strait.

The Strait was formed through glacial erosion of the bedrock and was flooded after the end of the ice ages. Before the Strait was dredged in the modern era, it was possible to walk across the Lavan Sands, located to the east of Bangor, at low tide. Llanfaes, the town King Edward destroyed to build Beaumaris Castle, was the largest commercial center in North Wales prior to the conquest, and it was located along this ancient pathway, which went from Holyhead, through Anglesey to Llanfaes, across the Lavan Sands to Aber (or Garth Celyn as it was known in the middle ages) over the pass known as Bwlch y Ddeufaen, across the Conwy River at Caerhun, and then east to Chester.

The Strait plays an important role in the history of Wales not only because of this pathway, but because of the physical barrier it presents between Anglesey and the mainland of Wales. The main reason for this are the dangerous tides which cause very strong currents to flow in both directions through the Strait at different times. One of the most treacherous areas is located between the current two bridges, known as the Swellies.

I’ll try to explain what happens in a way that makes sense: A rising tide–so a tide that’s coming in–approaches from the south-west, causing the water in the strait to flow north-east as the level rises. Keep in mind, that the tide is also flowing around Anglesey in a clockwise direction. After a few hours, this still-rising tide starts to flow into the strait in a south-westerly direction from Beaumaris. By the time this happens, however, the tidal flow from the southwester or Caernarfon end is weakening. Even so, the water continues to rise in the Strait until the tide turns and a similar sequence of events takes place, except in reverse. This means that slack water between the bridges tends to occur approximately one hour before high or low tide.

This dangerous tidal situation has made the Menai Strait a significant barrier for armies interested in taking over north Wales and was the site of one of the greatest Welsh victories during the 1282 war against England a the Battle of Moel y Don. In order to conquer the Strait, the English army had built a bridge of boats that stretched from Anglesey to the mainland. At the time, a man name Tany was in command. This is in November of 1282, during a lull in the fighting with ongoing negotiations mediated by the Archbishop of Canterbury between Prince Llywelyn at Garth Celyn and King Edward at Rhuddlan. Up until the Archbishop stepped in, King Edward had been working on a plan to catch Llywelyn in a pincer movement with him attacking Gwynedd across the Conwy and Tany from Anglesey.

Tany jumped the gun, however, and didn’t want to wait or maybe simply saw an opportunity. Regardless, it is understood that he didn’t consult the king before crossing his bridge of boats on November 6, 1282 during slack water. The Welsh were waiting for them and Tany was met by an overwhelming Welsh force on the mainland beach. By the time the English retreated back across the bridge, the tide had shifted.

From the English chronicles: When they had reached the foot of the mountain and, after a time, came to a place at some distance from the bridge, the tide came in with a great flow, so that they were unable to get back to the bridge for the debt of water. The Welsh came from the high mountains and attacked them, and in fear and trepidation, for the great number of the enemy, our men preferred to face the sea than the enemy. They went into the sea but, heavily laden with arms, they were instantly drowned.

Tany, several other nobles, sixteen English knights, and over 400 of other of Tany’s men died, while the Welsh had few casualties. The remaining English army made it back to Anglesey, but their losses were too great for them to attack a second time, not to mention they’d have to rebuild their bridge of boats.

From The Chronicle of the Princes: … The king and his host came to Rhuddlan. 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.

This battle set the stage for Llywelyn’s decision to travel to Cilmeri at the behest of the Mortimer brothers, Edmund and Roger, in hopes of turning the tide of the war to his side for good.

This battle is played out in much the same way in Cold My Heart, the first book in the Lion of Wales series.