The Torre de Belém is the main icon of Lisbon. It is a small structure in the Belém neighbourhood, touching the waters of the Tajo river. To access to it, you have to cross a short wooden walkway and when the tide go in, the tower looks like it is completely inside the water.
It was built as a defense tower because the mouth of the river to the sea is perfectly visible from it, and it was also used as a lighthouse. In 1983 it was declared World Heritage Site by the UNESCO, and nowadays, it received thousands of visits every year. What it is really worth to see is the sunset. The tower has 5 floors communicated through spiral staircase and it finished in an amazing terrace with views to the Tajo.