The 1st level includes the Palace of the Underworld, which leads to a labyrinth. The rest of the 1st level is pretty much ruins. I do not recall anything else about this level.
According to MAYAS and ZOCQUES Southeastern Mexico: Chiapas, ISBN 798-970-678-031-7, Page 42, (Laura Pescador Canton author), the Mayans started building the Palace of the Underworld around 500 A.D. The Palace contains 11 vaulted corridors, laid out in a labyrinth shape. The Palace’s layout is a ritual and symbolic reference to the underworld entrance, a place that the Earth Monster inhabits.