The Coatepantli is the restored "serpent wall" near Pyramid B, a carved enclosure with snakes, skeletal figures, and military imagery. It is easy to rush past on the way to the Atlantes, but the wall gives Tula much of its bite: this was not a soft ceremonial city.
The reliefs are best in angled light, when the carving reads more clearly. If you have seen similar serpent motifs at Chichen Itza, this is one of the places where the old argument about Toltec influence starts to feel less abstract.