In Barcelona there really is not much left of the old roman structures from the 1st to the 4th centuries AD. Typically there are just bits of walls and towers. You can find one of the more interesting of these roman ruins off Carrer (Street) Duran i Bas where there are three arches worked into a wall. These are particularly old and are the last supports of the aqueduct that brought fresh water into roman Barcino.
What is shown in this picture is a reconstruction of one of the aqueducts that carried water into the city around the 1st century AD. Here also you can see one of the original lateral passages for pedestrians and two semi-circular towers that defended a gate that is no longer in existence.
There is an explanatory marker across the street from these ruins that provide some detail about what you are seeing.
Roman brick can often be seen in the walls around the cathedral and also as part of the walls of the buildings that stand to each side of the small side streets and byways near the La Ramblas.
The Romans distinguished between those bricks which were merely dried by the sun and air (lateres crudi), and those which were burnt in the kiln (cocti or coctiles). Typically they preferred clay which was either whitish or decidedly red (like the ones I saw in Barcelona).
They considered spring the best time for brick-making, and kept the bricks two years before they were used. They made them principally of three shapes; the Lydian, which was a foot broad, 1-1/2 feet long; the tetradoron, which was four palms square, i.e. 1 foot; and the pentadoron, which was five palms square.
While you are walking around the side streets between the La Rambla and the Cathedral or in the Barri Gotic section be sure that you cast your glance upwards occasionally. Most tourists are looking into the windows of the stores or noticing the cobblestone streets under their feet. Few, however, seem to take notice of the interesting architectural and historical sights that are above them in the walls of some of the older buildings.
Up on the walls you can often see Arrow Loops (An opening in any type of fortification wall, usually shaped like a key hole, vertical slit or cross, that allows an archer to fire his weapon with a great amount of protection) where the ancient citizens manned these walls in defense of the old city. I found them on many of the buildings close to the old roman areas and the cathedral.
Occasionally I think I also saw some of the old battlements (a fighting position on the top of the castle wall or tower. This includes the crenellated wall and the wall walk) from which the defenders could fight off attacks.
Visit the BARRIO GOTICO (Gothic Quarter) at night, It's amazing...antique and dark...buildings are huge and grey... it's so mysterious... There is a square: PLAZA DEL REY, placed @ the old JEWISH neighbourhood; it is said that THE CATHOLIC KINGS welcomed COLUMBUS there when he came back from AMERICA.
I took a picture, but it was so dark nothing can be seen!