Nice build!
I have a hexagonal slab of plywood (I think) en route from Litko for cutting out hex-shaped bases for a modular cave system, so I am eagerly watching to steal borrow any cool ideas from projects like this!
I'm looking forward to seeing your work. So I can also
steal borrow ideas 
Very convincing beefcake!

"They have cave trolls!"
I'd be tempted to go lots of curvy tunnels as well as the "caverns" to make a more maze like system. You do seem to have a few tunnel modules I see.
Cheers
Matt
If I ever get around to buying another slab of polystyrene I'll definitely make some more mazelike tunnels, but I'm also thinking about making some more ruin areas, a big square and so on.
Next update:
All the floors are now brown. Thematically they would look better stone colour but the single colour tone gets quite boring so this helps add some variety.

And here are the passage blockages for the edge of the board. I should have put the stone colour texture on the backside as well so they could be used in the middle of the board as well. I can still do this though as I need to do something to the backs of them to make them stronger. The fit really well with the boards as well. Occasionally you need to find the right one to fit the spot but it's not that hard.

Next is tidying up the brown paint where it went on the grey walls. Not a hard job. Then onto the drybrush and washes (although the washes don't seem to work very well on this texture)
After that it's all the small details. Mushrooms and so on. 1 week left in my term break (with 2 days work during that time and a Birthday) so I should get the main part complete
