Area 15. Stone Gate
The title of this section is a little misleading. The text description includes both the Stone Gate and a series of pit traps beyond.
The Gate itself is a stone wedge, 4 feet high, ten feet thick, two feet wide at the narrow western end and ten feet wide at the eastern end. "It is impregnated with strong anti-magics which prevent its detection or removal or change to another form or substance." The text from Area 14(c) emphasizes this point, "It cannot be magically detected, nor will it open by physical or magical means . . ."
As noted in Area 14, there's a slot in the Stone Gate that's just big enough to fit a coin, a flat gem or a magic ring. If you put a magic ring in the slot two things happen (1) the Stone Gate sinks into the ground, allowing entrance to the areas beyond, and (2) your magic ring gets crushed to bits and is lost forever.
The module tells us that the Stone Gate opens easily from the other side, but it does not say if the Gate rises back up into place after the PC's pass through it.
Beyond the gate is a stairway down, and a hallway that runs to the west. The hallway is blocked with three doors, one after another. On the other side of each door is a 10' deep pit, just like the ones back in Area 3. The module suggests that anyone using force to open a door has a 1 in 3 chance of falling into the pit beyond.
There's a concealed door hidden at the bottom of the third pit. It's easy to find if you look for it, but the module assumes that the PC's will be expecting the third pit and won't look to closely at it.
IMO: The Stone Gate is where you "sacrifice aught but a loop of magical metal." The riddle wouldn't be hard to figure out if it was written somewhere in Area 14, but I think the distance between Area 3 and here makes it harder. My first instinct if I was faced with the Stone Gate would be to put coins in the slot. And if I'd managed to figure out the deal with the 3-armed Gargoyle back in Area 11, I'd be prepared to dump a lot of coins down there.
"Two pits along the way will be found to lead to a fortuitous fall, so check the wall." That's the second part of Area 15. Pass two pits and then check the wall at the bottom of the third. Again, the tricky part is making the connection between Acererak's riddle and this part of the Tomb.
Note that the pit traps are dangerous. Each pit is full of poison spikes. Anybody who falls in has a 50% chance of being hit by 1 to 3 spikes, and each spike requires a save versus poison.
Another gag here is the idea that the players will think the doors are stuck and use lots of shoving and force to open them, when really the doors open easily, dumping the PC's into the pit traps like Moe in a Three Stooges bit. But how likely is it that the players won't check to see if the door is stuck first? My players check every damn thing in the world before they open a door. If they ever make it this far, they won't fall for this one.
Also, there are only a handful of normal doors in the module so far, and the text doesn't say if they are stuck or not. Is there a default expectation that any dungeon door would be stuck?