Torre dos Clérigos isn't the tallest building in Portugal, it's the tallest in Oporto. For instance, Lisbon has some buildings over 100 meters high.

About Oporto, it's well worth a visit for at least a weekend. Apart from the magnificent historic center where my personal highlight is the Igreja de São Francisco (Church of São Francisco), whose interior is richly decorated with lots of golden objects and altars, you shouldn't miss out the cityscape from the south bank of the Douro river, the Foz district (where the river meets the sea) and the modern building of Casa da Música (Music Hall). Don't forget to eat a "francesinha", a typical sandwich made out of many different meats, cheese, a spicy sauce with Port wine, beer and many other "secret" ingredients. Delicious!