We had a quick look at the the most ancient part of Barcelona near the Metro stop called Jaume 1. On one side is the busy Via Laietana, on the other, the remains of the old Roman wall. Barcelona existed before the Romans built the fortified town of Barcino around 15 BCE, during the time of the Emperor Augustus. These walls are not visible at street level now. What one sees is the Roman wall from the 4th century CE and later additions.
We walked along via Laetana until we came to the Placa de Ramon Berenguer el Gran (featured photo). The Roman wall has been used here to prop up the medieval Chapel of Santa Agata. The arches and windows that you see here belong to the chapel, which dates from the early 14th century CE.
We walked along the impressive walls looking at the mixture of old stone replaced by later brick filling. Doors had been cut into the wall at some time, and one of these was impressively decorated in the modern street style (photo here). We walked along until we came to the remnant of what must have been an aqueduct supplying water to the walled town.
A longer walk would also have been interesting. It is also possible to visit the extensive archaeological discoveries under the Barri Gotic. We saw the entrance near the cathedral, but, regretfully, had too little time to do this.
In the rest of the Gothic Quarter of Barcelona it is hard to tell true history from fanciful reconstruction. The Roman wall anchors you to real history: the founding of Barcino, possibly by the Laietani, the arrival of the Romans a century or two later, and the eventual fall to Visigoths in the 5th century CE.