I LOVE Desktop Tower Defense!

All About the Towers 

• The Pellet Tower: These towers are what I consider the basic building blocks of a good maze. Their cost is low ($5) and they start out at 10 points of damage (upgrading to 400 points) but they are rather slow. I typically don't spend time or money upgrading these towers. Two good things about them: they hit both land and air targets and at their top level, they have a large range (they can basically shoot a pellet across the map!).
• The Squirt Tower: These are my favorite towers. They are fast and moderately priced (starting at $15). They upgrade to 450 hit points and are given the "very fast" designation. Best of all, they fire on both land and air. Good for killin'!
• The Dart Tower: I rarely use these because I guess I haven't discovered their secrets. They shoot far across the map and they are good at targeting groups of bad guys but they are very slow. Try them. You might like them. But I'm not too fond of them myself.
• The Swarm Tower: If you wish to survive the medium and hard levels, you're going to need at least three of these towers upgraded all the way to their peak. The only downside to these towers is that they don't work on anything but the fly guys. But they are incredibly effective at what they do!
• The Frost Tower: This is a handy little tower that doesn't cost much at all. It's goal is to freeze the bad guys (all but the Immune creeps) to give you more time to hit them with your strongest guns. They effect both land and air and even add a little damage themselves.
• The Bash Tower: An expensive tower that does a great amount of damage, especially to large groups (but nothing to air). I love to use these in the long games, like The 100 or the 10K Gold. At their peak, the deal out 2,000 points of damage!
• The Ink Tower: Another tower I rarely use. This one has a minimum and a maximum range, meaning that it's only going to shoot out globs at creeps that come within a certain distance of it and then that's it. Plus, they are slow, they only hit things on the gound and they only do 300 points of damage when fully upgraded. I'd rather use a Squirt Tower!
• The Snap Tower: These are one-shot towers that fire a large amount of damage to a number of creeps at the same time. I haven't learned their secrets either, so I don't know all their ins and outs. But what I do know is that they don't cost a lot to upgrade and that they do 3,200 when they're fully upgraded.
• The Boost Tower: So helpful when you are playing a long game, these towers boost the hit points of any tower they touch. For an investment of $500 total, you can raise any tower to be 50% stronger. I like to use these when I play the 10K Gold game.