There a few remnants of the medieval city wall, which once encircled the city, in the Ribeira area: east of the Dom Luis bridge and just west of the Praca da Ribeira.
When you get into town, just tell the cabby to take you here and it will be all you need to know. There are pubs beyond belief and just up around the corner are clubs for dancing.
The whole area around Riberia is really nice, you don't even have to go down to the river. Also the streets above are full of small shops and nice buildings. And the view is great!
The Ribeira is a really cool area. Just sit there and enjoy a coffee, cake, while resting by the Duoro....
Ribeira is the oldest district in Porto. The streets are narrow and filled with small shops, the houses higher and deeper than they are wide.. Here live the lower classes.