The old roman town of Barcelona was originally a basement settled by the romans 2000 years ago. It had 4 main gates with 2 main streets crossing from north to south and from east to west surrounded by the walls.
Today just few things remain of that ancient roman city of Barcelona, but still some of that initial urban layout can be seen. In the picture below we can see one of the main entrances of the city, just close to the Cathedral.
One quick anecdote that can be seen in the picture is the little statue in the left tower. It is a small image referring to Saint Roch, a catholic saint that is devoted to fight against the plague. In 1560 the city celebrated the festivity of this Saint because it helped to cure from the severe black death the city was suffering at the time.