Well, if you're ballsy enough to boast that a building is zombie-proof, it best be ready to hold up under a fine toothed comb and magnifying glass. People that have said as much, within the respective worlds of zombie-fic, have an annoying tendency to be WRONG. Undead wrong. *cracks knuckles*
Issue #1: The speed that that little turtle can pull in its shell. Those mechanisms better be FAST. There also needs to be a setting you have it at where, when the perimeter's proximity sensors trip (which I assume you have), starts the process.
Issue #2: Maintenance and power. Hope you're good at the first one and have plenty of the second at a nearby, secure location.
Issue #3: Garage?
Issue #4: No gun ports? The zombie apocalypse is all about active defense. Also take into account that not all your foes may be undead.
Issue #5: The wall is nice but a moat would make it even nicer. Pungie pits are also cool. (Sharpened wooden stakes at the bottom of a covered pit).
Hope these tips make your zombie apocalypse lifespan that much longer. Seriously, though, those windows could pose a problem for you.