Photo: Wikimedia CommonsThe double-deck iron bridge designed by a student of Eiffel is Portos defining image, linking the Ribeira quay to the port lodges of Gaia. Walk the upper deck alongside the metro for the widest views of the river and the terracotta rooftops, then cross the lower deck at water level. It is free and open at all hours. Time your crossing for sunset, when the whole valley glows, and keep a hand on your phone in the crowds.