I'd say it's a small city, more compact than anything on earth. Because there's more structures below the ground than above, since the sun doesn't shine above ground underground structures would be much more popular. Wouldn't you say? 

And the small area ABOVE ground is only around 3-4 miles wide, but underground it penetrates at least three miles of solid manmade structure.

I mean since we're going with Alien planet, why not go with Alien building techniques?