The Caracol at Chichén Itzá (circa 1050, Yucatán, Mexico) has a tower with two concentric walls and a circular staircase inside leading to a small room near the top of the structure. The windows along the staircase and the opening at the top were probably used for astronomical observation, which is why the Caracol is also known as The Observatory.
Tom Owen Edmunds/Bridgeman Art Library, London/New York