The Roman Theatre of Cartagena with the Rafael Moneo museum approach
Cartagena's first-century-BC Roman theatre lay hidden under the old town until 1988, and the only way in is a brilliant architectural sequence designed by Pritzker laureate Rafael Moneo: you pass through the Palacio Pascual de Riquelme, descend past Carthaginian and Roman strata, then emerge into the vast tiered cavea that once seated 6,000. It is the single most unmissable thing in the city and a five-minute walk from the pier. Few Roman theatres in the world are entered through a museum that tells the full layered story of the stone before you stand in it.