Belem Tower, the iconic landmark of Lisbon, at night. Constellation Taurus, the Pleiades star cluster, and Jupiter (bright point) appear on the left above the 25 de Abril Bridge across the Tagus river. Belém Tower or the Tower of St Vincent is a fortified structure which is designated a UNESCO World Heritage Site because of its significant role in the Portuguese maritime explorations in the 16th century. The tower was a defense system at the mouth of the Tagus river and a ceremonial gateway to Lisbon.