Porto, also known as Oporto in English, is the second largest city in Portugal.
I think the outstanding tower is the Clérigos tower.
Clérigos church tower was the tallest structure in Portugal when completed in 1763 (the national record is now Lisbon's Vasco da Gama Tower), and as the main feature of the city's skyline, ships used it as a guide when coming into Cais da Ribeira.
It's a remarkable baroque landmark designed by the Italian architect Nasoni, who also designed the adjoining church. It has become a popular tourist attraction for the aerial view of the city from the top (76 meters up -- after a steep flight of 240 steps). Inside the church, the highlight is a polychromatic Baroque-Rococo marble ratable.