I am pretty sure it was a story and not a novel. In the story, Conan and some soldiers are in a jungle I believe it was. They are attacked. The soldiers believe the attackers are supernatural, but Conan does not. He tracks one of the wounded attackers to a cliff wall. Up on the cliff is a bloody handprint which marks a secret entrance to a forgotten city. Conan tricks some guards there into escorting him into the city. The guards are imprisoned for allowing Conan to trick them. Conan is thrown into the lair of a creature. He survives and escapes, rescues the soldiers and holes up in a tower within the city. A large battle insues with multiple sides.
That sounds like the plot of the El Borak story, "Three-Bladed Doom," which de Camp rewrote as a Conan story, "The Flame Knife." It's in Conan the Wanderer, so have a look at that one and see if it's the story you're remembering.
I checked and that is in fact the one! I had passed over it because it did not start in the wilds as I remembered and the name was misleading to me, but it was the one. I am sad to say it is my favorite Conan tale as I remember it. I read them a long time ago, and maybe it is time to reread them and see if this still holds true. Thanks for your help!