Answer: It is surrounded by water on three sides. The royal navy was nothing to be messed with back then ( Still should not be taken lightly today) This leaves the city to bombardment by cannons. If the citizens withdrew they would pretty much let the british in, if they stayed they risk being crushed or injured by cannon.
It provided security and safety to the people from violoence and war during the fall of Rome by establishing a stable caste system. It was able to keep out invaders, and people were bound to each other by loyalty.