I retried and got to lvl 43 then quit.
It is a very hard game to do at first. But once you get the hang of it then it will be a piece of...VIRTUAL CAKE!
Now there are a few suggestions I have on how you could upgrade this...
1 Every 3-4 levels you get +10 hp.
2 Speacial powers, like, System crash (it would make all enemys stop for a short time.), pop-up blocker (allows you to choose some enemys and take 1/2 of their hp.), Pay-day (it would give you 2x the amount of money you get for each enemy for a certain amount of enemys.), DELETE (it would destroy 1 random enemy and give you 3x the amount of money you would normaly get for that 1), and of course all of them would need to be bought and would take a while to recharge.
3 A create a map mode, you would be able to make your own map, with as many rounds of enemys you like up to 50, set up starting cash, starting power ups, and sooooo on.
4 anti lag.
5 cheat code section, when you die at a certain amount of rounds in normal game play you will get a cheat code or two, 30-first two power ups, 40- all towers build will be at max power, all 50-infinate cash.
So there, I made a few suggestions that I hope you will use, and really this is a good game but gets boring at lvl 36...
SO READ THIS PLEASE!
If you don't I will get very mad, because this took me 15 min to write.
Good day Sir.!
^-^