I foolishly, FOOLISHLY, followed Darren's link to Desktop Tower Defence. I'm not providing the link directly to DTD because you'll follow it and then get addicted. You can go to a tiny bit more effort and read Darren's warning before you start mainlining flash gaming crack. Don't do it! Just say No!
Too many evenings after the kids have gone to bed have been consumed. It's time to stop. I managed to make it past 7000 on a wide variety of scenarios, including my favourite "wall of death squirts".
Four different configurations are
I like using a tactic of deleting and starting towers to pull the level 47, 48, and 50 creatures towards the exit, then routing them back to the kill zone. That's the funny line to the top right.
Jeremy has a strategy for 7000, and his consideration that it is harmful from 2 months prior.
I agree! It's time to stop. Until I figure out how to get to 7200...