The Belém Tower was built both as a ceremonial gateway to Lisbon and as part of a defence system of the entrance of the Tagus river and the Jerónimos Monastery, which was necessary to protect Lisbon. The system was initiated by King John II (1455-1495), who built the Fortress of Cascais and the Fortress of São Sebastião of Caparica. The banks of Belém were protected by a ship, the Grande Nau, replaced by the Tower of Belém during the last five years of the reign of King Manuel I.
http://en.wikipedia.org/wiki/Bel%C3%A9m_Tower
The Belém Tower was built both as a ceremonial gateway to Lisbon and as part of a defence system of the entrance of the Tagus river and the Jerónimos Monastery, which was necessary to protect Lisbon. The system was initiated by King John II (1455-1495), who built the Fortress of Cascais and the Fortress of São Sebastião of Caparica. The banks of Belém were protected by a ship, the Grande Nau, replaced by the Tower of Belém during the last five years of the reign of King Manuel I. http://en.wikipedia.org/wiki/Bel%C3%A9m_Tower