Mayans were known for human sacrifice. The belief was a sacrificed person went immediately to their version of heaven instead of battling through the nine layers of Xibalba, the Mayan underworld, to win their way into heaven. Fail at any of the levels was to be stuck there for eternity. These flat round bases at the foot of the stones that look like grave markers are where heads were lopped off.
The pyramid itself has 39 steps (I counted them). The number 13 (and multiples of 13) is the numbers of heavens so is also a sacred number to the Mayas. We learned this from our quite funny guide, a descendant of the Maya, who grew up in these parts prior to the ruins being exposed by excavation. To climb to the top of this pyramid is, in essence, ascending to heaven.