Desktop Tower Defense is a crazy-good, crazy-addictive game. I used to play this game all the time, but I never actually put any effort into winning. Yesterday I beat Easy and Medium for the first time, and got to level 50 in Hard. (Of course I also played Easy yesterday for the second time ever.)
If you're stumped on how to win, go on YouTube and google it. You'll find a gazillion short videos of winning strategies. You can also check the DTD winning maps page for winning strategies in their final form.
Go to DTD and play in Fun/10K Gold mode. This will give you the ability to precisely re-create any winning strategy you want, tower by tower. But it doesn't actually give you enough towers and upgrades to guarantee a win. So, you copy successful strategies, and then see how long you can stay alive copying somebody else's strategy. This makes it easy to figure out what makes the strategy successful.
Then copy that strategy in a real game.
Then modify the strategies you're copying, because they won't work for you exactly. I did this and pretty soon ended up with a unique strategy that had nothing to do with the models I was copying. To make those models work, I found I needed sniper towers; after a few more games, my system for winning revolved entirely around sniper towers and long mazes, with a little support from centrally-placed typhoon towers, plus a few anti-aircraft towers and freeze towers.
(By the way, the essential uselessness of the above description really demonstrates why beating DTD is a killer use case for YouTube. Sometimes showing is much easier than telling.)