suggestions/help needed to balance TD map (please?)

I am having trouble balancing difficulty in my TD map (available here):
http://filebox.vt.edu/users/aaronz/war3/(8)AZTowerDefense-Locked.w3x

I used a spreadsheet to try to make sure the towers do a fair and reasonable amount of damage based on their cost but a few users have complained that the map is too easy even on the hard setting. I have trouble beating it on normal so I assume that I am just a crappy player.

At any rate, has anyone else had a problem like this and if so, how did you figure it out? If anyone would like to test out the map, please feel free to download it and offer suggestions. I will be happy to give credit to anyone who helps and I will also be happy to help you out if you are a map maker and need help with some triggers (assuming I know enough to help).

Thanks!
-AZ

It doesn't sound like your towers are disbalanced to each other so much that they are too strong versus the running units.

The best solution to a problem like this is to test the map exhaustively. Make small changes to all your towers until they seem to be doing adequate damage in general, and then start tweaking individual towers that seem too weak or too strong.

It will take forever, and it will still be a little imperfect at the end, but this kind of process is what makes certain that the map is playable and fun. Without that certainty, you'd be wasting the effort you've already put in to the map.