The largest example of Gothic architecture in the world, the Catedral de Sevilla took centuries to complete and is the burial place of a handful of iconoclastic Spaniards, most famously Christopher Columbus. The cathedral is an exceptional work, inside and out, and has an outlandish eighty chapels dedicated to Roman Catholic prayer. It's tower (Giralda) is quite a climb, but worth it for the excellent views of Seville. The church is open daily, with an entrance fee of 8 euro (about $11 US). Even if you don't go inside, the cathedral will often be a fixture as you explore the Andalucian capital's downtown area and can be viewed along with other renowned attractions like the Royal Alcázar of Seville, the Archivo de Indias, and the incredible Plaza de España during a day of walking.
Note:Dress well, for they will turn you away if you are in a t-shirt or sandals.