The City's Secret is the eighth chapter of Among Thieves. After finding the correct temple marked on Marco Polo's map, Nathan and Chloe left Elena and Jeff at the entrance while they explored its contents.
Inside the temple Nate and Chloe quickly caught sight of a massive statue, holding a representation of the Cintamani Stone, as well as various objects in its other arms. Nate recognized the statue and looked inside his journal for notes, which indicated that the statue's objects and arms needed to be in specific positions. After he moved them the 'stone' split open, revealing a triangular hole, which Nate revealed was a lock fitted for the Phurba; which acted as the key. This drained the nearby pool, revealing a hidden passage underground.
While heading through the passage they spotted a wall with a stone carving of a tree, and wondered what it's connection to the Cintamani Stone was. As they progressed further, they fell through a trap door and were met with a ceiling made out of a descending spike trap. Nate managed to save them both by destroying the trap's gears, disabling it entirely. Shortly afterwards they stumbled upon the hidden area's main chamber, containing an enormous Phurba dagger replica.
As the chapter begins head into the chamber where the giant multi-armed statue is. To proceed to the next area the player has to position the statue's arms to match the drawings Nate has in his journal, which the game tells you to open with a prompt. Climb the broken pillar on the right side to ascend to the upper walkway, then head left to adjust the statue's left arms. The trident is already correct, but you do need to climb up to the dorje to turn it over. Jump down afterwards and head the other way, using the bars above the doorway to swing to the other side. This walkway will partially collapse halfway through, giving you a convenient path back up. Adjust the arm holding the shrunken heads, then climb up the one holding the phurba dagger and jump onto it to finish the statue.
Once the 'stone' has been opened you can climb the statue's chest and press to insert the key. Follow Chloe down the newly revealed passage until you eventually run into a trap, which you can break by shooting the teeth on the large stone gears behind you, until the gears stop catching on each other. If you have no ammo there is a loaded MP40 near where you landed, although a well-thrown Mk-NDI can accomplish the same thing. With the ceiling no longer descending, climb out and head down the hallway, and end the chapter by entering the next chamber; starting "Path of Light".