The modernist route
Barcelona is definitely a mother city for Modernism. And do you know why this trend is so well represented in Catalonia? The country was really open-minded to the trends coming from the rest of Europe in order to reinforced its nationalism and its differences with the rest of Spain.
Thus, the ideas of many architects and artists were easily accepted as bases of an artistic renewal.
The most representative artist of Modernism is of course Antoni Gaudi, and you can admire several of his works throughout the whole city. Others big masters like Domenech I Montaner and Puig I Cadefalch are also famous.
The Quadrat d'Or (the Gold Square) in the Eixample district shows some marvels. There you can admire the Casa Mila, also called La Pedrera. It was sculpted with wave forms and the roof looks like the body of a ship.
La Manzana de la Discordia (Bone of Contention) gathers 3 houses and each owner uses the genius of one architect to build their house.
Further you can find the Casa Calvert, the first edifice of Gaudi, adorned with angels faces, wrought iron balustrades...
A bit further away from the city centre you will find Park Güell, another master work of Gaudi. It was first supposed to include private houses, administration buildings and leisure spaces. It was supposed to be an English style garden city but those plans were never finished. In the whole park you can find the familiar curves and mosaic.
The masterpiece of Gaudi is without any doubts the Sagrada Familia (the Holy Family), a modernist house of God, never consecrated by the Church. Gaudi died before its wind-up and despite the polemic about letting it as it was, the constructions keep going on. The Sagrada Familia has 3 façades : the Passion, the Nativity and the Glory of the Christ.
This is a real stunning building. I was staying for a while in the Hotel Sagrada Familia in Barcelona, a few years ago, and I was always so astonished when I arrived at the turn of the street in front of this fascintating building...