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. Construction of 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. The water in ditch that surrounds the castle was supplied by the sea. The two gatehouses of the inner court are large in size with two round tower at the inner corners containing circular staircases. Each gatehouse is protected by large semi-circular towers on the outside. A large hall occupies the first floor of the northern gatehouse and most likely would have also occupied the southern gatehouse as well but this has been destroyed. In the tower that is situated in the center of the eastern curtain wall inner ward has the remains of a chapel above a crypt, both being vaulted. Protecting the southern gatehouse is a square barbican. Its design prevents a large scale attack on the gatehouse and would leave attackers in a small space easy to target with missiles. As mentioned above, the outer ward is surrounded by and eight-sided curtain wall roughly rectangular in shape. The walls and towers of the outer walls being much smaller than those of the inner ward. The outer ward has two gatehouses, one to the south and one to the north that was never completed. To the eastern side of the southern gatehouse is an extension to the outer curtain wall. This has an internal passage and looks to have been designed to protect the area where ships would have docked to resupply the castle. Between 1403 and 1405 the castle was occupied by the forces of Owain Glyndwr who rebelled against King Henry IV.