eaumaris Castle was built by Edward I and construction began in 1295 on the Isle of Anglesey after Edward put down the revolt of Madog ap Llywelyn. The castle was built on marshy land overlooking the Menai Strait and positioned to control the stretch of water between Anglesey and the mainland. Edward gave the job of designing the new castle to his master castle builder James of St. George. James designed a concentric castle consisting of an inner rectangular ward with six towers and two gatehouses surrounded by a octagonal outer ward having twelve towers and two smaller gatehouses. The outer section was started a couple of decades after the inner sections but parts of the gatehouses were never completed. To the side of the southern outer gatehouse is a dock where ships with supplies could unload. It was important that the castle could be resupplied by sea as Edward could not guarantee that regular supplies could be sent through the hostile Welsh countryside. Between 1403 and 1405 the castle was occupied by the forces of Owain Glyndwr who rebelled against King Henry IV.