St Mary of the Sea Basilica, Barcelona

Apart from the Cathedral and Sagrada Familia, this is the most visually appealing religious building in the city. You should include it during any visit to the Barri Gòtic (Gothic Quarter). It's an austere Gothic structure with a large and impressive 15th-century stained-glass window in the shape of a rosette. Built between 1329 and 1384, it features some outstanding sculptures on its facade and in the tympanum. Mass is conducted in Catalan and also in Spanish on certain days. On Sundays mass is held in the main altar; otherwise it's in the Santísimo chapel.