Hovenweep, solstice and the Anasazi

Tower functions are subject to speculation, as they have limited access, contain few windows and many have narrow slots or peepholes placed in the walls. Towers are often linked to a kiva, generally through a tunnel, suggesting they may have been used for ritual functions. The slots and doors of Hovenweep Castle, in Square Tower Group, have been shown to define an apparent solar calendar. The building is aligned so that light is channeled through openings into the building at sunset of the summer solstice, the winter solstice and the spring and fall equinox.

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