Barcelona is the capital of Catalonia and Spain's second most populated city: around 1.7 million inhabitants in the municipality and more than 5.7 million in its metropolitan area. It is the seat of the Generalitat and the main economic engine of the Spanish Mediterranean.
The city sits between the sea and the Collserola hills. It was founded as Roman Barcino in the 1st century BC and later became the capital of the County of Barcelona and of the Crown of Aragón. Barcelona is the city of Catalan Modernisme (Gaudí, Domènech i Montaner, Puig i Cadafalch): Sagrada Familia, Park Güell, Casa Batlló, Casa Milà, Palau de la Música. It is also the Gothic Quarter with its medieval grid, La Rambla, El Born, Gracia and the Barceloneta seafront. Montserrat and Girona make perfect day trips.
Barcelona has a complicated relationship with tourism (more than 10 million visitors a year), but it remains one of Europe's most vibrant cities. Top-tier Mediterranean cuisine, vermouth at noon, calçots in season, and street life that runs late.
Three or four days is the sweet spot. The best time to visit is May-June and September-October: avoid August (locals go on holiday, plus heat and humidity) and the worst cruise-saturated months.