This is actually pretty cool and the system works out well. I suggest making the player duck when he is behind cover. If you developed this and added enemies/missions/weapons and such this could be fantastic!
Barrels don't hurt you. This means that at a high enough level (expert starts high enough) you can select barrels and run by the hoard. Inevitably, some demon will set it off but if you keep throwing down barrels, the chain keeps following you. Then, run INTO the zombie hoard leaving death in your wake. Fun stuff.
My score went off the screen, try shrinking the text. Otherwise, the music is pretty awesome. Fun for 2 minutes but needs something more. Something to make it unique.