El Caracol
o the north of Las Monjas is a cockeyed, round building on a large square platform nicknamed El Caracol or "the snail" for the stone spiral staircase inside. The structure, because of its unusual placement on the platform and round shape (the others are rectangular, in keeping with Mayan practice), is theorized to be a proto-observatory with doors and windows aligned to astronomical events, specifically around the path of Venus as it traverses the heavens.