The Eixample is Ildefons Cerdà’s 19th-century bet that a city could breathe: a chamfered grid, plane trees, and enough pavement to wander without a plan. At its eastern edge the Sagrada Família no longer looks like a work in progress so much as a finished argument with gravity.

Step inside mid-morning or late afternoon. The stone columns still branch like trees. Red, gold, green and blue glass still flood the nave. What is new is the height — 172.5 metres of it — and the knowledge that Gaudí’s central lantern is finally doing the job he drew.

From there, walk Passeig de Gràcia. Casa Batlló and Casa Milà (La Pedrera) sit a few blocks apart, mosaics winking in the same light that is currently overheating every photographer in a 200-metre radius. You do not need a costume for this. You need something light, cotton, and willing to be seen in.

South of the grid the city folds in on itself. El Born and the Barri Gòtic are a medieval argument against straight lines: Santa Maria del Mar’s sober stone, Roman fragments underfoot, and plazas that appear only after you have already given up.

Lunch is not a reservation so much as a sequence. Pà amb tomàquet, garlic prawns, patatas bravas, a glass of house vermouth. El Xampanyet on Carrer de Montcada still pours cava the colour of old gold. Plaça de Sant Felip Neri keeps its pockmarked walls. Plaça del Rei keeps the light.

This is the neighbourhood that rewards a tee you can sit down in, spill a little olive oil on, and still wear to a rooftop later. Barcelona is not a cocktail-dress city. It is a cotton-and-espadrilles city that happens to have a gothic cathedral.

Park Güell was supposed to be a housing estate. It failed at that and succeeded at being a fever dream: serpentine benches in smashed tile, gingerbread gatehouses, and a terrace that lays the whole city out like a tablecloth, sea at the far edge.

Below the park, Gràcia still behaves like a village that never agreed to join Barcelona. Plaça del Sol, Plaça de la Virreina, independent shops, and a Sunday vermut culture that does not care you have a timed ticket up the hill.

Go late. The mosaics at golden hour are the reason the photographs exist. Then walk down into Gràcia and stay for the squares. If you are in town in early June, Primavera Sound is the city’s other cathedral — louder, dustier, and equally convinced of its own mythology.

Barceloneta was a fishermen’s quarter. It is now the city’s coastal living room: a grid of low houses, rice restaurants, and a promenade that runs from the W Hotel past Port Olímpic until the light gives up.

Order a bomba — the neighbourhood’s fried potato-and-meat cannonball, brava and allioli on top — and do not let anyone charge you theatre prices for pan con tomate. As evening drops, the air finally cools. This is when a slightly heavier cotton starts to make sense, and sangria stops being a cliché and becomes a temperature management strategy.

Walk the boardwalk. The city heat is behind you. The Mediterranean is doing what it does. Barcelona is one of the few European capitals that can end a Gaudí day with your feet in the water.