 During the Hundred Years ' War a French attack ( 1340 ) burned a manor house and took some prisoners , but failed to get into the town . In 1403 the town was burned by Breton raiders . In the late fifteenth century , Plymouth Castle , a " castle quadrate " , was constructed close to the area now known as The Barbican ; it included four round towers , one at each corner , as featured on the city coat of arms . The castle served to protect Sutton Pool , which is where the fleet was based in Plymouth prior to the establishment of Plymouth Dockyard . In 1512 an Act of Parliament was passed for further fortifying Plymouth , and a series of fortifications were then built , including defensive walls at the entrance to Sutton Pool ( across which a chain would be extended in time of danger ) . Defences on St Nicholas Island also date from this time , and a string of six artillery blockhouses were built , including one on Fishers Nose at the south @-@ eastern corner of the Hoe . This location was further strengthened by the building of a fort ( later known as Drake 's Fort ) in 1596 , which itself went on to provide the site for the Citadel , established in the 1660s ( see below ) . 
