To the north of the nunnery is a cockeyed, round building on a large square platform. It's nicknamed  "El Caracol" ("the snail") because of the stone spiral staircase inside. The structure with its unusual placement on the platform and its round shape (the others are rectangular, in keeping with Maya practice), is theorized to have been a proto-observatory with doors and windows aligned to astronomical events, specifically around the path of Venus as it traverses the heavens. (from wikipedia):

http://en.wikipedia.org/wiki/Chichen_Itza