These are just observations, NOT written in stone...

I would compress my air from a windswept source, bubble it through water[which would
keep fish alive], and install a floor drain at the lowest point-which might allow heavy gasses[Argon, Methane, Whatever is heavier than air] to drain away so you can breathe].

Angling the walls of your trap-door spider hatch inward should direct the spent rounds up and back[giving your attackers something to worry about].

Many times Man copies nature, I would create a water egress route and perhaps connect
to existing sewers/basements/ruins.

Of course having a "bat cave" we could drive into would be nice, I used a storage facility on
my daily route for just that reason.

During my rebelious[sp] youth, I had a blue Honda civic tricked out in neon green after-market products that commited sue-ah-side[phonetic] on the highway out of shame.

Were I young again, I suppose I would rent a small bobcat-bulldoser, find a spot near a
huge rotted tree stump and a river, and dig my tunnels to intersect at the main room.

Were I rich, I would weld semi-truck containers together like Legos, re-enforcing with
concrete and rebar, and covering everything but the exits with a foot or two of topsoil,
Probably using concrete and rebar re-enforced six-foot culverts for the entrances.

Your friend, the sasquatch.