Mercat de Sant Antoni stands on the gate it replaced. When Barcelona's walls came down after 1854, this iron hall went up on the rubble of the Sant Antoni gate, finished in 1882 by Antoni Rovira i Trias and Josep Maria Cornet. It is shaped like a cross with an octagonal dome where the arms meet, and it fills one whole block of the new grid, with four pavilions turned to face the cut-off corners. It was drawn for the city being built around it rather than the one just demolished.
A rebuild that ran for nine years dug the floor out and found what the market had been standing on. Go below the shopping level and there is a bastion of the seventeenth-century defences, ninety metres of it and five metres high, with its ditch beside it. Running alongside is a paved branch of the Roman road that came down the coast from Rome. Under both is a shelter dug in 1937 for people hiding from air raids. Three layers of the same instinct, stacked, and on a scheduled visit you can walk between them.