With the ladder on the top of the picture it looks more like some sort of tower that has been blown over and now is sitting on its side.
The ladder has a curve on it as if it used to be a vertical ladder that let you get to the top and hold on as you climb overt the edge.
The bit the troops are entering looks like it used to be a roof hatch that had its own sloped cover hatch.
EITHER THAT or it's the biggest barrel of bear ever made and the guys are in to mop of the dregs!!!!!!!!!

Nige