El Caracol - Observatory

It is not entirely clear what the real purpose of this unique building was at Chichen Itza, but the general consensus is that it was an astronomical observatory. It helps, of course, the fact that the dome looks like an astronomical observatory. What is clear is that the Mayans came to have some very important astronomical knowledge (a lot more than European cultures at the same time, for example) and so this is very likely to be the building's use.