Thanks to the Discoveries of the 15th and 16th centuries, Lisbon became the world’s main centre of commerce. In order to protect the city, King João II and King Manuel I devised a pioneering system of maritime defence. For this project, in the narrowest part of the Tagus bay (1,670 meters), D. Manuel I had the Tower of Belém built in 1514. As a result, crossfire between the two banks could be created to block the entry of enemy ships.