Dear Gawd...
This is by far one of the best defense games I have ever played. Perhaps even the best flash game to have been created yet.
Firstly the graphics are stunning. You've really captured the zombie horror feel with the intricately animated intro sequence the detail on the walking corpses and the wall. Everything is superbly done.
Gameplay is perfect for this style of game. Having incorporated the same style as Thing-Thing and Madness Interactive is a stroke of genius. Especially since I use a laptop, I can hold back the waves of the dead without it making the game difficult to interact.
The only problem I have, as I think a few people have noted, is the reloading. Not just the reload time, but the inability to interrupt the reload process. I would have loved to just drop the M4 and quickly switch to the Shotgun quickly. However, this DID make the game even more exciting.
Literally, every piece of this game is perfect. Great Job. Hope you get a high placing in the portal.