Above the fake sky of the military camp, if one should pierce through its illusory veil one would come to see a construct hanging from the underground ceiling like a Chandler.
But unlike a chandelier, mighty chains made of adamantine were crisscrossed around each other attaching to the building at various angles, ensuring stability to the building so that it wouldn't wobble or be affected by weight as it stood strong and undaunted.
As for how people entered and exited this building, that was unknown, but regardless if one were to look through the transparent walls of the construct what would come into sight would be a group of individuals sitting around a round table they all seemed focused watching a three-dimensional cube as it depicted various battles happening beneath them.
As for who these individuals were?