Eventually they settled upon shooting the buttons again.
It took a few tries, but the halfling shot the middle button and the room rose up, grinding and clicking, back to the 50' down position. The adventurers dropped a torch to verify.
On the platform, for a brief moment, the adventurers could see an undead monster --
...then it quickly snuffed out the torch!
They could hear it grunting and groaning below their feet. The group got a little panicky...
Since these are new players, I put on my "helpful DM hat" and told them to calm down. They were in control of the situation -- at least for now. The monster was 50' down in a pit with no apparent exits. What did they want to do?
PS: I got the order of the floors WRONG in my description yesterday. This map accurately describes the known layout under the ruin!
They decided they would try to raise it up to the 25' mark. Then the cleric would try and turn it!
Some more shooting took place and the leftmost button was hit. The floor moved up...
The cleric stepped to the edge of the doorway and forcefully presented his holy symbol. He rolled a d20 and got a 17! Success! The undead monster was driven back through the archway...
The hobbit and the cleric lit up torches and dropped ropes down to the sunken floor. The cleric went first so he could hold the undead at bay. The hobbit followed. He spent some time picking up their "misses" (arrows and bolts) from the floor of the room.
"How do we raise the floor all the way up?"
The adventurers explored a bit more and discovered another "control panel" just inside the archway. It was close enough that the characters could stand in the room and push the buttons on the wall. The floor doesn't "shoot up" so there's enough time to pull your arm back before the floor moves.
This time, the first set of runes were different --
They decided to push the "new" leftmost button.
With a grind, the floor moved upward to its starting level at the top of the stairs.
The adventurers settled into the room for the night, spiking the door behind them. They even brought the mules and pony in so that they'd be safe too. They hobbled them on one side of the room.
They set up watches and got some needed rest. Off to the mine in the morning!