You ask to look through a gate and examine one of the tunnels. It is a long dark shaft sloping gently upward and to the left. Given the lack of mechanisms or resources, it makes sense that they are not able to have a second gate placed within the tunnel itself, but perhaps they could put down something to warn them ahead of time which tunnel the lizards are approaching.
You suggest your early warning system to the Princess. She explains, with the translator's help, that they tried methods like that before, but the lizards have a light touch, can climb along the sides of the walls, and will cut down any ropes that might make noise.
In short, they can circumvent any early warning system that's been tried before.
The Princess instructs you to remain with the next group of guards, along with the translator. She then moves off to another group, and you find yourself on guard duty.