Battle of Moel - y - don The Battle of Moel - y - don was a battle fought in 1282 war during the conquest of Wales by Edward I . Background . After stripping much Welsh land from Llywelyn ap Gruffydd in 1277 , Edward I of England imposed harsh terms on the lands he had conquered . He had been aided in 1277 by many Welshmen , including Llywelyn ' s own brother Dafydd ap Gruffydd , but the terms of the peace , coupled with domination by English laws and by Edward ' s officials drove many of them into rebellion . The war began when Dafydd turned against the English and slaughtered the garrison of Hawarden Castle . Edward raised several armies through levy and indenture , and sent them into Wales on multiple fronts to surround and destroy Llywelyn ' s armies . One important expedition , consisting of 2000 infantrymen and 200 cavalrymen under the former constable of Gascony , Luke de Tany , was sent to capture the island of Anglesey . This would deprive the Welsh of much of their grain , and outflank the Welsh who were defending the Conwy . The battle . After successfully capturing Anglesey , Luke de Tany ' s men constructed a bridge of boats across the Menai Strait which separates Anglesey from the mainland . Edward ' s plan was to cross the River Conwy while de Tany crossed the Menai and attacked from the north . However , de Tany ignored the plan , believing he could defeat the Welsh without Edward ' s aid . He had contacted some of the clergy in Bangor who had promised to aid him by giving a signal when the time was right to attack . The bridge was finished in September that year , and on 6 November , de Tany and his men crossed the bridge having been given the signal to attack . Llywelyn ap Gruffydd had been alerted to the crossing somehow , and emerged with a large army to meet the English as they crossed . A rising tide cut off de Tany ' s men from the bridge . When they tried to flee , many of them drowned when their heavy armour dragged them under the sea . Walter of Guisborough wrote an account of the battle : 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 . Luke de Tany , the nobles Roger de Clifford , 5th Baron Clifford , Phillip and William Burnell ( brothers of the chancellor Robert Burnell ) , 16 English knights ( and their esquires ) and 300 of de Tany ' s men perished . The Welsh suffered few casualties . The remaining English army made it back to Anglesey , however their losses were too great for them to launch a counterattack . Aftermath . This victory , and the Battle of Llandeilo Fawr in which another English army was destroyed in South Wales , boosted Welsh morale and set back Edward ' s plans , but Llywelyn was killed a few weeks later at Cilmeri , having led an expedition into mid - Wales to rally supporters .