Complex sentence: HM Fort Roughs was one of several World War II installations, that were designed by Guy Maunsell and known collectively as "His Majesty's Forts" or as "Maunsell Sea Forts", and its purpose was to guard the port of Harwich, Essex.
Split into simpler sentences: HM Fort Roughs was one of several World War II installations that were designed by Guy Maunsell and known collectively as His Majesty's Forts or as Maunsell Sea Forts. The purpose of Fort Roughs was to guard the port of Harwich, Essex.