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 maritime system of defence for Lisbon. As part of 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.
King Manuel I wanted the Tower of Belém, located at the entrance to the kingdom’s capital, to clearly assert his power. Therefore, the king ordered his heraldry to be ostensibly displayed on the tower. These include the Royal Arms, his personal emblem the "Armillary Sphere", and the symbol of the Order of the Cross of Christ (which he administered).
As well as defending the maritime access to Lisbon, the Tower of Belém also protected the Jeronimos Monastery. This monument was originally ordered by King Manuel I who after Vasco da Gama's return from India obtained the funding available to turn it into his personal mausoleum. Later, the Jeronimos Monastery became the mausoleum for his royal lineage: the "Order of Aviz'.