In Desktop Tower Defense creeps don't follow a path. They only find the shortest way from the starting to the ending location. You are free to build a labyrinth with your towers in order to extend their way.
The game's got 5 levels of difficulty: Easy, Normal, Hard, Challenge and Fun. Personally, I play on Normal mode most of the time. After a few tries you'll probably figure out a strategy that works for you. My top score so far is 5540. Post your top score in the comments if you can beat me. I really advise you to give it a shot. Sure you will love it! PLAY Desktop Tower Defense.