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.

In kilometers, how long was the Plymouth Sound Breakwater?