Here's some tips if anyone wants them. It's up to you if you want to follow them:
gunmen are incredibly important and awesome past 20. A good number to have is around 60. Though feel free to go past that (I got to 140 eventually)
don't buy any gun except shotgun and the gun you will use endgame (generally that's old glory)
shield towers are useful in late game. Don't get 3 though as the third makes little difference.
getting the right stuff early is important. For example, upgraded your wall should be left till later as it shouldn't be getting hit in the early levels
Your main fire power late game will be your gunmen and (sometimes) your towers. Keep that in mind
Laser towers are better than missile turrets from middle to late game as your army of gunmen will take out anything that's weak
Repairmen won't seem to important from day 20-35-ish if you're playing right, but it's good to stock up for levels ~38+
Have fun:)
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