Your game is no longer fun. Remove the programming for SdKfz's on waves 1-5 maybe?
Oh wait, what am I saying? Four riflemen with less than 80 rifle between all of them can clearly take an armored vehicle with a heavy machine gun.
Love to see a Tower Defense game arise from the Bubble Tanks creators. Not only is it a great game just alone, but it has quickly risen to the top in the games I play on Kongregate. Admittedly, it is an extremely hard game to beat (And I mean to get the "Bubble Bubble Toil and Trouble" Badge, but in being so, it is far better and more addicting than most tower defense games before it - Finally, a game it doesn't take me a single afternoon to beat. Thank you Hero Interactive.
Make a version where you can turn off the sounds. I mute it and it plays. Again and again. Also make a longer one, but other than that, IT'S FREAKIN AWESOME MAN!
[+] if you like the idea of a mute that works.