When Portugal was a leading seafaring nation, the Belém Tower welcomed explorers and merchant vessels to the port of Lisbon. Today, it's a tourist attraction with a fantastic view.
The tower was commissioned by King John II. It replaced a heavily-armed ship that had until that point patrolled the estuary. A twin tower was built on the other side of the bay where enemy ships could come under fire, but this was destroyed by an earthquake.
The tower has served as a prison and a customs house. It was declared a national monument in 1910 and in 1983 it was classified as a UNESCO World Heritage Site.