It would be more interesting to rig up a sticky piston underneath the block in front of each door. The flip of a switch will block all doors so zombies can't break them down. Might as well make the best out of the situation.
I was also considering this, but zombies can actually break the door with only one block in between, it would have to either cover the whole door or cover the top. The latter isn't as easily hidden.
I was also considering this, but zombies can actually break the door with only one block in between, it would have to either cover the whole door or cover the top. The latter isn't as easily hidden.