The Hidden City – Sixth Monolith

After you return from the CENOTE and receive the Serpent Guard disguise from Uchu, Lara will at last be able to enter the Upper City. As you make your way toward the point where you're supposed to meet Unuratu's son, Etzli, you'll pass this proficient Yucatec monolith (6/7). (Check the annotated level map if you're having trouble finding it.)

Reading the monolith reveals a clue: "They stand back-to-back, two brothers in arms. I lie at the feet of the one who watches the shaded serpent."

Selecting the monolith on the in-game map reveals the area in which the hidden treasure can be found.

Continue up the stairs to the north...

...to find the Upper City Main Gate Base Camp.

Continue northward, past a large statue built between the two sides of the stone staircase. This statue is one of the "two brothers in arms."

The second "brother" stands on the next landing, with his back to the first. He faces the carved snake's head mounted above the doorway ahead. And, sure enough, the monolith riches...

...survival cache (23/34), are buried at the second statue's feet.