The Belem Tower was built in the 15th century as a defensive tower guarding the entrance to the Tagus River in Lisbon. It was part of King João II's plan to build fortresses in a triangle formation at the mouth of the Tagus for protection. The tower is decorated with Manueline symbols representing the king's power. Over time, it served various functions including customs control, a telegraph station, and lighthouse, and also as a political prison. It was classified as a UNESCO World Heritage Site in 1983.