Great game. Really fun to min-max and find the best strategy for it. I wouldn't recommend aiming for higher after you get past the survival mode 6 million achievement award though, the higher you get the more annoying it is. I don't think you can get more than 8 million if even 1 tower dies in survival mode (setting you back a huge amount of time).
My strategy summarized:
1-Abuse AI targeting. You only need four turrets with max (6 armor), one for each corner of the screen. Try to always have 1-3 health packs per armored turret, so you can have breathing room to not panic-switch to health packs, and instead focus on building more turrets.
2-In survival, armor isn't too useful until 3 million ish because turrets can heal from upgrades.
3-My damage turrets are range-range-range-lightning-lightning-anticapital. My meatshield towers are 6-armor meatshields. Every other damage turret, I use range-range-range-amplifier-lightning-anticapital instead, so that most or all of my towers are damage boosted. The damage boost seems kind of weak, but the upgrade takes very little time for the forge to make, so it balances out.
I wish you could place turrets and stuff while the game is paused.