This is actually a rectangular platform about 6 feet high, 890 feet long and 30 feet wide upon which the heads of conquered enemies were piled to show them off to the people.
Here's what it says about it on one site I found.

The Wall of Skulls is called the Tzompantli, which is actually an Aztec name for this kind of structure, because the first one seen by the horrified Spanish was at the Aztec capital city of Tenochtitlan.

The Tzompantli structure at Chichén Itzá is a Toltec structure, where the heads of sacrificial victims were placed; although it was one of three platforms in the Great Plaza, it was according to Bishop Landa, the only one for this purpose--the others were for farces and comedies, showing the Itzá's were all about fun. The platform walls of the Tzompantli have carved reliefs of four different subjects. The primary subject is the skull rack itself; others show a scene with a human sacrifice; eagles eating human hearts; and skeletonized warriors with shields and arrows.

Party on!