I like this game, but it gets old fast
This game is definitely a lot of fun at first, and certainly a challenge. However, once you figure out the "trick" it quickly becomes a bore.
I know this was already mentioned but when you use a combination of the turtle tower and the power up tower the enemy stops completely. Obviously this makes continuing with the next 30 rounds completely pointless as no enemies will EVER make it to the organs.
Even before I figured out the freeze glitch, I got a self sustaining defense together before the last rounds came around. Another strange glitch was with the 'next germ' marquee at the bottom. I saw several interesting looking opponents, but some of the coolest looking ones came out as the little white peons you get at the beginning of the game.
Overall I think it has a lot of potential, and if you bumped up the surprises, maybe added a few more germs that did different things (maybe even attack the towers or temporarily disable specific towers). There's waaaaaaay too many rounds in the later stages.
All in all it consumed about 40 minutes of my life. Nice job!