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Photo: via Wikimedia Commons, CC BY-SA 3.0
Romanesque monastery church dating to the 12th century when the Augustinians took over from the Benedictines, but largely rebuilt in the 16th. The cloisters have 42 columns and depict Biblical scenes.
The Collegiate church and cloister of Santa Juliana is a collegiate church located in Santillana del Mar, Spain. The church is dedicated to Juliana of Nicomedia.
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Museo Nacional y Centro de Investigación de Altamira
The caves of Altamira, discovered in 1868, have prehistoric paintings as old as 36,000 years - they were added to or amended over another 20,000 years. The condensation from 20th century tourists damaged the paintings so the caves were closed to the public and a replica cave built - this is what you tour, along with the adjacent museum. The true cave is 500 m north but closed off. The tickets can be bought via the official page or on the spot - but in the latter case, expect you may be waiting 3h or more get in.
Cistercian Monastery / abbey built in neogothic style 1906/10 and now run by their branch order the Trappists. You can attend mass and buy their famous cheeses.
Monte Castillo is riddled with caves containing Paleolithic art about the same age as Altamira. Two caves are open to view, and here you see the originals not a replica. But that means tightly restricted numbers, so you need to book ahead, you're unlikely to find availability on the day. It's worth seeing both: Castillo, and Las Monedas named for coins found there.