Well the main tips: build narrow labyrith; build area towers on the turns of the labyrith; build splash tower between two ways of the labyrith; compile splash with slowing area and damage boost and you'll get great mix for killing simple, split, slow enemies; try to build all the towers in one place to easily destroy regeneratives; use machinegun with range boost to kill the bosses and spawn enemies; build one splash ghost in the middle and simple ghost near the exit; build mega towers that are compiling good enough, for example compile range boost with area, damage and slow to get a wide slowing area attack, build some weakers near this tower and you'll get a great damage; well, that's what i got atm.
It's a really easy game, just make a long path and get sniper towers with damage enchancers next to them, they will snipe them all off while they go through a long maze.
The problem is mainly balancing out paths with upgrades. How long should a path be before you can safely upgrade? How many towers should be upgraded. It's obvious that you need to make a long path, however some levels aren't that easy... unless you are using the cheat obviously.