This is what I'm thinking of. We scour the region for supplies and blockade all the entrances that can be reached from the ground or by a short climb. then we leave the loading dock usable. (its got 3 of them) There is enough space at the loading docks for a number of heavy vehicles.
We mount snipers on 6 hour rotations on the crenellations on each corner and plan out the seperate floors for use as an enclave type community. We only go outside in force and well armed to scavenge for supplies, ammo, more vehicles and what not.
Maybe I'm overthinking this?
