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Arguably the most interesting part of the ruins - this was the much older political center of the city, from around the same time as the Roman Empire. It's centered around the "astronomical complex": three side-by-side temples that the sun rises behind on the winter solstice, equinoxes, and summer solstices respectively, as viewed from a nearby pyramid. The pyramid itself is surprisingly well-preserved for its age, showing striking, huge "masks" on each side, because it was actually enclosed within a larger pyramid whose structure was removed by early archaeologists.