Did you know there is a city that can be visited under the current Barri Gòtic? Inside the Barcelona museum, you can visit the old roman ruins that are left. Still many streets and building walls and columns can be seen in the Barcelona History museum (MUHBA).
In this museum you can visit also the big hall (Saló del Tinell) where the explorer Christopher Columbus was received at his return of his first trip to America in 1493.
Fascinating Museum, defintely worth visiting.