As a hint, don't buy any upgrades before wave 4 :) just use the money to place turrets and initially place them close together to get though the waves quickly. I think if you can get to Wave 5 then it scales in your favour and you should be good! I would like to build a mechanic to refund say 50% of the turret cost but it was just too far a stretch with the time. I actually ran out of time with a fairly long list of things I would still have loved to implement.
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great effort :) Wave 10 is a monster! I find that I only place towers as I need to in the rounds to collect as much money as I can. I only place a new turret if a zombie breaks through. Then I tend to balance my upgrades mainly on speed and damage, however, I get enough range so that if you see the middle 2 rows that the back row of towers has enough reach to attack with the front row of towers......if that makes sense. but yes Wave 10 is pretty hard.