You can find these elements of Roman Barcelona at these points in the city. The city was founded around 15 BC.
Via Sepulcral - Plaça de la Vila de Madrid
Aqueducts - Carrer de Duran i Bas and Plaça Nova
The arches of one of the aqueducts are integrated into the walls of a 19th century building. Water was transported from the river Besos and the Collserola Serra and from wells, cisterns and 2 aqueducts.
North West Gate - Plaça Nova
The towers form part of the Bishop's palace now and the Deacon's house.The current name is Portal del Bisbe, it was the Praetorian Gate. In the 4th century, all 4 gates had 3 openings, the large central one for vehicles and the side smaller arches for pedestrians. Only the 2 decumanus gates still exist. The cardus gates, pulled down, were in the 'call' or jewish quarter and in Plaça de l'Angel.
Domus - Carrer de la Fruita and Sant Honorat
This is 4th centry
Temple and Forum - Carrer del Paradís
The temple was dedicated to Augustus, placed at the highest point of the city, in the forum,which was to be found somewhere under or near Plaça Sant Jaume (or possibly Plaça Sant Miquel nearby - they can't excavate so they don't know, as i understand it). The 4 columns and what's left of the temple can be found at the above address.
Workshops and factories - Plaça del Rei
In the musuem here,there are the remains of a laundry (fullonica), a dye-works (tinctoria), a garum factory (cetaria) and cellars.
North East wall - Plaça de Ramon Berenguer el Gran
South East wall - Carrer del Sotstinent Navarro
The first century BC foundational wall had a few towers and a defensive moat. The second wall, built in the 4th century was placed in front of the old wall, greatly thickening it and 76 towers were added. The towers were cylindrical on either side of the gates but mostly square. There was a moat at least along the Avenguda de la catedral of 6 metres depth.
The Sea Gate - Carrer del Regomir
What can be seen is one of the smaller pedestrian entrances. It is thought there might have been a castellum here.
This is the route to follow the Roman walls:
Plaça Nova, Avinguda de la Catedral, Carrer de la Tapineria, Plaça de Ramon Berenguer el Gran and Plaça de l'Àngel, Carrer del Sotstinent Navarro, Plaça dels Traginers, Carrer del Correu Vell, Carrer del Regomir, Carrer d'en Gignàs, Carrer d'Avinyó, Carrer dels Banys Nous and Carrer de la Palla. A large part of the structure disappeared during the 19th century (such as Palau Reial Menor, Convent de l'Ensenyança, Plaça de l'Àngel, Plaça del Regomir) though in some cases, like Pla de la Seu, the destruction was prior to this.
Taken from this recommended website:
http://www.bcn.cat/museuhistoriaciutat/ca/muhba_placa_del_rei.html - click on Guia Barcino/BCN to see the pdf
This image i found at this website