Yah, I was wondering..
If that flying skeleton thingie destroys a barrier, can you no longer repair that barrier? I found that trying to defend the whole house was worse than useless, so I just walled up the four doors in the bottom left room, that left a bunch of zombies uselessly milling about in the upper room, which was really useful for cutting down the volume, and that way they could only come from one direction as long as I protected the barriers. I got to level 20 without any problems, and then a flying skeleton made it to one of my barriers and once he destroyed it (in one hit!) I couldn't rebuild that barrier.
Recommendations for play: buy no turrets or bombs until you've maxed weapon AND barricades. Max weapon first, upgrade barricades as needed so they hold long enough for you to get to them before they're broken down. Once you've done that and you have enough money saved up you can build four turrets if you want and keep replacing them when they run out. I'm gonna go see if that works on the levels other than normal. Woot!
I was getting a little tired of the game once I maxed weapon and barriers, after that it was pretty much just grinding, so it was wise to end it at level 20. That's just about optimum for a little challenge without making it too repetitive. Excellent graphics, sound, gameplay. I have no criticisms. Well, more upgrades and achievements are always nice but I think this is great as is.