Belem Tower was built in the 16th century from limestone as part of a defense system on Tagus River and was a built as a ceremonial gate of Lisbon. This, however, is not the only reason why this tower is so famous and why it remains a UNESCO World Heritage site. The Belem Tower is a symbol of the Portuguese maritime travels during the Age of Discoveries.
More about this image here:
https://sumfinity.com/hdr-photos/portugal/lisbon/belem-tower/
