To get anywhere in level 6, I had to experiment a lot and look at forums. Here are some things I figured out:
Splitters pretty much need area damage from SAMs to be dealt with. Early game, going for a short maze, a fully upgraded red SAM, and one or two mid-upgraded DCAs dead center of the map will give you enough firepower to get to the early 30s. It's intensely satisfying to see entire waves knocked out with one or two missiles.
Other info:
1. After building a short maze, focusing on upgrading one or two turrets all the way to Red (rather than building more or upgrading more halfway) is usually the best way to go. Damage doubles at every upgrade, so you get 1x - 2x - 4x - 16x - 32x - 64x firepower from a single square. Red Plasmas do INSANE damage to individual targets.
2. An exception to this - purportedly, Vulcan turrets are most efficient when left at Level 1 in terms of raw damage. So building a HUUUGE maze of nothing but white Vulcans will also get you a long way in the start. Thinking up different, more efficient maze layouts is one of the funnest parts of the game for me. Radiators, spirals, and intestines are good inspiration.
3. SAMs and Red Vulcan turrets are great because of their range. The more often individual turrets can be kept firing, the more damage they'll output while things are on screen. The best maze layouts have most turrets firing most of the time.
4. There's a risky trick you can do to keep enemies in your maze indefinitely. You must sell an earlier turret to make a gap in your maze, then block off the normal exit. Enemies ahead of your gap will turn around and march back through all your guns. You can then seal that gap off and send them marching back along the normal route, repeating until they're toast.
5. Freezes and Sonic towers give an extra bit of 'oomph' needed for later waves, but don't seem necessary early on. With both, placing one or two of them where they'll hit the same wave of enemies two or three times seems to work really well.
6. DCAs eat air for lunch. They do one thing and do it very well.
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