The wind is sharp, but the towering walls offer a useful shelter, and the snowfall is light. For a minute or two the only sounds on the air are the rattle of rusted panels as the wind whistles through, and a distant bark of gunfire.

There's an entrance to the wall ahead. We're not far inside, you'll hear the noise. I think we have most of the attention for now. Just watch out for snipers: there are a lot of shadows in here.

There is indeed an open doorway ahead, once pressure-sealed and now twisted back on weather-pitted hinges. Beyond that are metal stairs that circle up and through the massive outer wall to a cavernous interior bridged and lined with walkways in better or worse repair. At least the big floodlights above are on, high above.

Guttural shouts echo down from the left. Then the harsh clap and reflected light of a flashbang grenade. Heavy impacts. The high whine of arc rounds being fired.