For those interested in history the National Museum of Roman Art is a must visit; this was the first museum outside Madrid to be designated "national" by the Spanish government, and you can see why. The building was designed by Rafael Moneo, its stone arches evoking the construction of many local Roman buildings.
laid by Roman masons 1900 years ago:
The Proserpina aqueduct is based at a dam which is located about 10 km north of Merida. It is the youngest of the three aqueducts of Merida and was build in the time of Traian at the beginning of the first century AD. A feeder aqueduct brings additional water into the reservoir of the dam, which collects the water from the Proserpina aqueduct. The dam is 427m long, 21.6m high and at the crest 2.3m wide. It is a earthen dam supported on the inside by a 6m thick retaining wall. This wall is built from a concrete core and granite masonry facing. The dam is built from three segments of slightly different orientation. On the inside nine buttresses support the retaining wall of the dam when the water is drained from the reservoir; without these buttresses, the retaining wall would collapse due to the pressure of the wet earth in the dam embarkment behind it. It has been suggested that is was the collapse of another similar dam, that of Alcantarilla for the aqueduct of Toledo, which brought the Roman engineers to build this safety feature into their design. On the outside, there are also buttresses, 16 in total, supporting the retaining wall, but these are covered by the earth embarkment and only known from drilling.