Experiment, and try out different combinations of towers. I suspect that it's best to have a fairly tightly wrapped maze, to take advantage of each tower's range. I've had a fair bit of luck trying to wrap the maze around the base itself.
For the stealthed enemies, you pretty much need at least one of the spike towers, but you can also place walls which will reveal them when they attack, allowing your towers to fire on them. I've never had much luck with the fire towers, and I don't build the arrow towers except for 1 at the very start. Though, that might just be a product, or maybe even necessity, of my maze.
Later on, I've found that I absolutely need lightning towers, and I've found that I can do without missile towers at all. Using the maze I use, I've been able to reproduce about the same score (between 4.4 and 4.9 million) with a fairly wide variety of towers chosen.
To me, the maze seems to be the most important bit.
Hope this helps! I'm certainly not the best, and I may have just gotten lucky, as I am having a heck of a time trying to find another maze design that scores well.