As quickly and quietly as you can, you race back to the gate and notify the guards of what you've seen. The excitement and fear is palpable. None of them want to face down the Big One directly. Talking quickly, they come up with a plan, calling over to guards at a nearby gate. After conferring for several moments, they set forth.
The men at the far gate open theirs and rush into it, heading for a side passage that connects to the tunnel that the Big One is currently traveling in. They're working to get behind it without it realize it. Once it's gotten close enough that there are no more side passages it can sneak away in, they'll seal off the tunnel.
You and the men where you are work to seal the gate. There are large stones cut into cylinders for this purpose. They're easy enough to roll around on one side, but can be raised up onto a flat, making moving them exceedingly hard. It takes fifteen of you to stand up the cylinder, but once it does, the gate will not be able to be opened.
You start to wonder why this is not done on all of the gates, but realize that sealing themselves in would only result in them all starving to death, as there are no natural food sources inside the cavern, just the water.
Even as the men start to relax at having sealed the gate, you think you can hear the foot steps of the Big One approaching. For a creature so large, it can move fairly quietly. Just the conversations of the men are enough to drown it out, so you hush them. Through a narrow crack in the stone you see it walk right up. It then looks through the crack, right back at you.
For what feels like a long time, the two of you stare at each other, unblinking. Then, a rumbling echo races down the tunnel, and the Big One looks away for a moment. When it looks back, you are smiling.
With some difficulty, the Big One turns itself around and begins making its way back down the tunnel. As it does this, the men return from the far gate, celebrating. The tunnel has been sealed on both ends. Anything inside it is trapped now.
The lizard assault eventually does come, but they are poorly organized and the men are able to route the without great difficulty. For several hours the Big One roars and rages against the gate, but even its strength was unable to move the stones that sealed it in. Everyone celebrates, for you have won.
The End.