Rome's presence on the Iberian Peninsula began during the Second Punic War in 218 BCE, when the Republic moved to counter Carthaginian power in the region. The conquest was slow, taking until 19 BCE to complete. The Romans initially operated from Emporion and Tarraco, but at some point during this period they occupied the Iberian settlement at Montjuic to control the mouth of the Llobregat River. The new colony of Barcino -- probably a Latinization of the Iberian name Barkeno -- took the form of a Roman castrum, with a cardo maximus running along today's Llibreteria and Call streets, a decumanus maximus following Bisbe, Ciutat, and Regomir streets, and a forum near the present Placa de Sant Jaume. The grid was modest, the population between 3,500 and 5,000 at its peak in the second and third centuries.

Barcino's first wall was simple: few towers, gates only at the four cardinal points of the perimeter. The Praetoria gate stood at today's Placa Nova, the Decumana gate on Regomir Street, the Principalis Sinistra at Placa de l'Angel, and the Principalis Dextra at Carrer del Call. But when Franks and Alemanni began raiding Roman lands from the 250s onward, simplicity was no longer enough. In the fourth century, the Romans rebuilt the fortifications on a different scale: a double wall two meters thick, the gap between the layers packed with rubble and mortar, and 81 towers rising about 18 meters high. Most towers had rectangular bases, except the ten semicircular ones guarding the gateways. The perimeter ran 1.5 kilometers -- compact, dense, and formidable.

The walls outlasted the empire that built them, but not unchanged. After Almanzor sacked Barcelona in 985, the city added castles to protect the four gates: Castell Vell, Castell del Bisbe, Castell de Regomir, and Castell Nou. As medieval Barcelona expanded beyond the Roman perimeter, King James I allowed buildings to be constructed against the wall in the thirteenth century -- and permitted windows and openings to be cut directly through it. In 1260, he ordered an entirely new wall encircling the growing suburbs. The Roman fortifications, now fully embedded in the city fabric, slowly disappeared behind newer construction. Much of what survived the medieval period was demolished during nineteenth-century urban modernization.

The surviving fragments are scattered through the Gothic Quarter like archaeological punctuation marks. At Placa Nova, two towers from the Praetoria gate still stand alongside a section of perimeter wall and an arcade from the Roman aqueduct that once carried water into the city. Along Carrer de la Tapineria, a polygonal tower that formed the enclosure's northern corner served as a support wall for later buildings. In the Placa de Ramon Berenguer el Gran, medieval walls sit directly atop Roman foundations, and the Chapel of Santa Agata perches on the ancient base. A circular corner tower survives on Carrer del Correu Vell; another stands at Placa dels Traginers. Smaller fragments hide inside private buildings on the streets of Regomir, del Call, Banys Nous, and Palla. In the basement of the Museum of History of Barcelona, excavations reveal not just the walls but entire Roman-era buildings -- a subterranean city beneath the medieval one, beneath the modern one.