While you're at it, make a thick steel floor so the zombies can't dig their way in (can zombies do that?). Although, if there is a near-infinite amount of zombies that can be re-reanimated as many times as necessary, they will probably get in eventually no matter what you do. I would suggest you use the time you have locked in a building to build and program an army of robots. Robots vs. Zombies. Neither is alive, robots can be rebuilt, and zombies can be re-reanimated. Robots have no brains to eat, zombies have no mechanical parts to salvage (unless they invented mech zombies). However, robots can use guns, while most zombies that I have heard of can't. P.S. Make it a very complex lock. Don't want any undead theives picking it and letting the hoard in, now do we?