 The 1 @-@ mile @-@ long ( 2 km ) Breakwater in Plymouth Sound was designed by John Rennie in order to protect the fleet moving in and out of Devonport ; work started in 1812 . Numerous technical difficulties and repeated storm damage meant that it was not completed until 1841 , twenty years after Rennie 's death . In the 1860s , a ring of Palmerston forts was constructed around the outskirts of Devonport , to protect the dockyard from attack from any direction . 
