A tiny, scruffy, much-loved tapas bar two minutes from Puerta del Sol, in business since 1860 and serving essentially two things: bacalao rebozado (battered chunks of salt cod, fried fresh to order) and croquetas de bacalao (cod croquettes). You queue at the takeaway hatch on the street, hand over a euro or two per piece, and eat standing on the corner.
Inside is a barrel-vaulted dining room with old tiles, marble tables, and the original gas lamps. It is one of the genuinely classic Madrid bar experiences — the surroundings, the procedure, the unchanging menu — and remains absurdly cheap. Beer €2.50; bacalao €1.50 a piece. Always rammed. Open every day from breakfast onwards.