A city in the Underdark

You manage to stay just behind the drow and your group. She's using two of her spiders as rear guard though, so you can't get too close.

After a while, you come across what looks to be a proper road, with multiple paths converging on it. Why would there be an actual road down here?

A couple minutes later, you enter through another cavern entrance, but this one is much larger than any other you've seen. As soon as you step out the other side, your hand shoots to your mouth, to not gasp at the sight.

Inside of what is probably the largest cave in existence, there lies a city, that is glowing in an eerie, blue light. With spiraling towers, seemingly wrought from blue stone, and a large fortress in the middle.

There are walls and city gates and everything! Where's the camp with just a couple sleepy guards, supporting their tired bodies on their halberds, while dimly scanning their surroundings for nothing? Listening for the sound of a well placed rock thrown down a cavern, ready to investigate an obvious distraction with a lame: 'Well I'll be! What was that?'

This is NOT what you bargained for. Not in the slightest.

But still, you are free, and your friends are not. You're their only hope, so you gotta get in there.