<constrained by...>
As I understand it, from something OMB said, if the sides and a top mesh was better formed, it could be useful for constraining the camera as the PC neared the entrance. That is, it'll keep the camera inside the cavern with the PC instead of letting it get lost behind walls and things...
But all the mesh should, I think be connected, sharing edges, not just added in with gaps between...
<...ignorance>