Do you mean Surge Walls?
I just checked my map and my wall is 6-7 deep with a row of Meltdowns (all water cooled) and Overdrive Projectors spaced enough to cover everything on the front lines. No point in doubling up on Heal Projectors since I don't think they stack healing. In fact I mostly just us repair bots since they repair quick and can go anywhere. I have about 14 of them but the die kind of quick so that's the only downside.
Also, I found Force Projectors to not really be worth the effort later in the game. Better to have more weapons to kill the enemy...the faster they die the less damage they can do. (In the below, image have one near my front line, but that was a test and I think I'll be removing it and putting the Meltdown back in it's place.)
I also have about 40 Revenant gun ships I keep flying around a relay point just behind the front line and send to attack once the enemy gets close enough to fire at the first wall. I find they help draw fire away from my buildings and they are much cheaper/easier to replace than Meltdowns and Surge Walls (titanium and silicon only).
I'm on wave 602 on Desolate Waste to give you an idea. It only gets dicey when I forget to release my Revenants on the "Heavy Cruiser Wave" (not sure what else to call it) that comes everything 6th wave.