If you're wondering which tower modification is the best - it's the Cannon. The normal Gun has instant bullet arrival but lacks AoE capabilities for the later zones. The Rocket is much like the Bazooka weapon, however over the massive distances of the 'assault position' areas, the considerable time the rocket takes to arrive at your cursor makes it very difficult to use effectively. The Cannon is the best of both - AoE that arrives instantly.
You may have to have sufficient fire power for this, I used the Ion Flux (if so just farm level 19) but go to level 24. It's extremely easy - hardly any harmful shots going around, and masses of enemies flying in a constant, predictable pattern. Just go to the left or right slightly from where you start to maximise your shots hitting the ships.
Go to level 19. Shoot at the line of ships for a few seconds until the powerup is about 1/3 down the screen, then dive into the stream, grab it and go to the bottom left. Try your best to dodge the chaotic salvo of shots then once the area opens up, go to the instinctive large area - sort of 3'o'clock to 6'o'clock. Consider using a smart bomb when you see the most enemies on screen around this time. Will net you up to $1000 a go.
You don't actually have to buy something from one tier to be able to purchase something in a further tier, unlike most ability trees. In my opinion the best thing to do is to save up 6 upgrade points without buying anything, then spend them on the Refractory Layer (lazer immunity). After playing a while you'll quickly realise how annoying and difficult to keep track of all the lazers become so it's a real blessing to have this.
I quickly recognised a possible symbiosis (strengthening and living off of each other) between the Deathcraft and Sharpshooter perks. I maxed them early and used only Automatic class guns, which fire quicker so will have more chance to active the perk.
The final weapon: the Gauss Rifle is worth saving for - because it has a high base critical hit chance, enemies, especially those with larger health pools, over the course of the level, will fall much easier.
I took the gun route of 10mm-SMG, AK-47 (maxing reload and some damage), Personal Minigun (maxing damage, reload) and then saved money for about 2 levels and went straight for the Gauss Rifle.
Try to buy the 1st grade of Turret and Helicopter as soon as you can.
You may not even need any Repair or Health upgrades until around level 20.
Prioritise killing the tougher enemies and leave the measly ones for your turret, picking them off at your leisure.
Lots of this might be fairly basic but hope it might help a few people. :)
 "Rogan the Swordmaster"){kind=icon}
 "Decision"){kind=icon}
 "Battle Beavers"){kind=icon}
 "Space Arcade (the game!)"){kind=icon}
 "Omega Box"){kind=icon}
 "Bloody Sunset"){kind=icon}
 "Liferaft: Zero"){kind=icon}
 "Stonepunk"){kind=icon}
 "Infectonator : World Dominator"){kind=icon}
 "Warzone tower defense"){kind=icon}