As you can see the trail is a mix of lava rock and a few occasional sandy spots. You can see more of the long and thick wall behind Mia. The lava walls were built in the 15th century.

“The walls of the temple are a study. The same food for speculation that is offered the visitor to the Pyramids of Egypt he will find here - the mystery of how they were constructed by people unacquainted with science and mechanics...Yet some of the lava blocks quarried out, brought over rough, broken ground, and built into this wall, six or seven feet from the ground, are of prodigious size and would weigh tons. How did they transport and how raise them?”
— Mark Twain, 1866