Since the city is built onto 2 cliffs on either side of the river, this Eiffel Tower-style bridge is a defining feature and a must see. Itself very cool, the view over the city also incredible.
Paris has the Eiffel tower we have D. Luís bridge. Contemplate this wonder of engineering from the XIX century from Gaia side and cross the upper deck to see a wonderfull view of Porto.
A project of the engineer Teófilo Seyrig, the "Luis I" Bridge was inaugurated on 31st October 1886. The bridge is built of two metal trays supported by a great iron arch and five piers with span 391m.
If you think it looks like the Eiffel Tower, there’s a good reason for it. Cross the bridge at the top for a great panorama of both sides of the river.
This is a must when you are in Porto. It has two layers. One is up to the top you can go and walk of which I did not but the one close to the river I walked many times. Lovely sights of the city.