Belém Tower, a UNESCO World Heritage Site in Lisbon, was built in the 16th century on the northern bank of the Tagus River. It was constructed during the zenith of the Portuguese empire. Portugal and its Iberian ally Spain were the world’s foremost naval superpowers during the Age of Discovery. The historical period began the centuries-long era of European colonialism, which eventually touched every corner of the planet. Click the above image to enlarge, click to view my high-resolution photo gallery, or click to read my feature article “Lisbon: A Warning to the Free World.”