The Vault is a central stronghold that offers a heightened covered bastion from which infantry can make a last stand against an encroaching foe. The vault proper has no ready access from the outside save through the upper bastion. The only way into the vault is through a subterranean tunnel. The vault will usually be protected by 4 to 6 watch-towers with heavy weaponry, including AA emplacements.
Design: I found some old bait containers near the river shore on an archaeological survey, and I immediately saw the makings of one or more buildings. I initially thought of turning the lid into an upraised square/park/fountain platform, but then I flipped it over and put it on the bottom, and this is the final result. The bait container was green Styrofoam, so I had to do a lot of brush painting, starting with a dark gray and then working my way up through the grays to a near white.
