Torre de Belém, Lisbon.
(UNESCO World Heritage Site)
This monument is a popular tourist attraction and an interesting piece of historical naval architecture.
It was originally built as part of a defence system and ceremonial gateway into Lisbon (for those coming from the sea, along the 'Tejo' river, obviously)
The tower was built in the early 16th century and is a prominent example of the Portuguese Manueline style, but it also incorporates hints of other architectural styles. The structure was built from lioz limestone and is composed of a bastion and the 30 m (100 foot), four storey tower.
It has incorrectly been stated that the tower was built in the middle of the Tagus and now sits near the shore because the river was redirected after the 1755 Lisbon earthquake. In fact, the tower was built on a small island in the Tagus River near the Lisbon shore.