 Before the latter half of the 18th century , grain , timber and then coal were Plymouth 's main imports . During this time the real source of wealth was from the neighbouring town of Plymouth Dock ( renamed in 1824 to Devonport ) and the major employer in the entire region was the dockyard . The Three Towns conurbation of Plymouth , Stonehouse and Devonport enjoyed some prosperity during the late 18th and early 19th century and were enriched by a series of neo @-@ classical urban developments designed by London architect John Foulston . Foulston was important for both Devonport and Plymouth and was responsible for several grand public buildings , many now destroyed , including the Athenaeum , the Theatre Royal and Royal Hotel , and much of Union Street . 
