Tidal Islands

I saw a picture on Bing.com last week of St Michaels Mount in Marazion Cornwall England.  It's a fortified island off the coast only accessible by a causeway that is underwater at high tide.

That would make a great zombie survival fortress to hole up in, and it's not the only such castle of this sort extant.

And there is another one with the same name in France.  

You got zed piling up at your wall?  No worries.  In less than 4 hours the tide comes in and washes them all back again.  You'd save a lotta ammo.  AND you keep access to the mainland if you need to sortie out on a patrol for supplies.  Boat access could also be a thing that would foil the shamblors.

It's a layered defense as well.  If zombies breach the lower wall at low tide, you can still be safe in the castle on the hill.  At high tide, zombie 'reinforcements' are cut off, making roundup and cleanup easier.

Corpse disposal of retired undead is simplified by mother nature as well, making the local environment more bearable for human survivors.