The Belem tower is the most popular attraction in Lisbon, at least according to some sources claiming it to be the most popular object in Lisbon to make pictures of. It is located very close to the Jeronimos Monastery and together with the monastery it was made a UNESCO World Heritage Site back in 1983.
The tower was built in the 16th century and it is a beautiful example of the traditional architecture style, and the somewhat strange style used during this period, the so called Manueline style. The tower has had many functions throughout history, it has been use to defend the area, but it has also been used as a prison and as a customs house. Today the Belem tower is packed with tourists from early morning till evening. If you want to see the tower at its beauty without big crowds, then that can be done from a river cruise for example.
To get to the Belem Tower you can travel with tram 15 or with bus 727, 28, 729, 714 or 751. Just like with the Jeronimos Monastery, do not forget to taste the Pastel de Nata in the Pasteis de Belem as you come visiting this area.