Wha?

You wake up in a strange building, surrounded by towers of glass. You are suspended on some sort of metal catwalk high above a vast pit. The walls are too high to jump, and too smooth to climb, but you can see the sun setting in the distance. Your head pounds, and you think you can see something through the murky glass. You hear footsteps on the metal, and you see a figure approach. Through the glass you first make out the shape of the body, then the face. It can't be! Your worst enemy approaches. You glance around and spot a shotgun lying nearby. You don't know how either of you got here, but you sure as hell won't both leave alive.