The space directly before the war bunker was simply a brick room (seen previously on another blog post called war room skeleton) before a skybox was added. A skybox, in conjunction with a sky camera allows a panoramic sky-scape to mapped onto the area in which a 'skybox' texture is applied. The sky camera itself takes its reference from the origin of the map allowing larger buildings and objects to be framed in a space further away from the main map. The reference camera uses 1/16 scale building to reduce the polygon count as well as requiring less skybox to render.
Fire entities and smoke entities also add to the war-like effect. Here some entity parenting work needed to be done to tie the gas to the canisters and effects like the canisters exploding when picked up.