Remember the secret Tower level in Warcraft 3?

Here's a stupidly addictive Flash version:
http://novelconcepts.co.uk/FlashElementTD/

Can't get past level 35.

You pretty much need to do the bare minimum to pass each wave so that you get the benefits of compounding interest. Then you'll have lots of money for the tougher waves at the end.

Level 39

That's a pretty cool game. I didn't realize you could upgrade towers the first time... got all the way through the second time.

What do you mean got all the way through?

The last level in the game (unfortunately :( ) is level 39.

Another Desktop Tower Defense game!
http://www.handdrawngames.com/DesktopTD/

Arg, that's addicting. It took me a while before I started getting a good strategy down. Died at level 49. :P

The last four or so waves in Hard mode are bastards :(

Here's another one:

http://www.candystand.com/play.do?id=17995

Edit: and another one...

http://ec2-1.playr.co.uk/onslaught/