What makes Chichen Itza a great example of Mayan engineering?

In addition to being a political and economic center for
the Maya, Chichen Itza also served as an astronomical and religious center. One of its
buildings, El Caracol (literally, "the snail" in Spanish, due to the remains of the
circular staircase atop the building) has been discovered to have been used as an
astronomical observatory. Although angles of the building mark the sun's extreme
positions during winter and summer solstices, others indicate the motion of the planet
Venus, which was considered sacred by the Maya. Although much younger than Stonehenge,
El Caracol demonstrates the same process of cosmic recordkeeping defined by a stone
structure.  To build a structure that could precisely define astronomical alignments
implies a sophisticated study of engineering and architecture.