How to beat it on hard

[mcast12]

The only turret that will work on any map is the 7 gold one, I've tried every possible combo and you can't win without them. Only ever upgrade them to level 4.

The cliff map...
Start with the 2 rows coming from entrance till you get to the cliff then double back (i added 2 aa towers to beat the first boss, after that you can sell theyre useless)
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You need to build the lasers asap, level 23ish. Upgrade the central row after you have a U shape, this one U shape should take you to around level 45.
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After 45 you need to build extra towers to give the laser more time for the final bosses.
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The river map...
Just use the 7gp towers again and do a vertical zigzag on the entrance section, when it hits the river do a sort of U shape to the base with the creeps running along the bottom.
You need to keep upgrading along the middle where the flyers come in. Get the lasers asap and build a few upgraded additional turrets along the middle, it's another easy win.
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The dune map...
This one is tough and you sometimes lose no matter what. Start with 2 lines of the usual 7gp following the roads up to the base, you need to follow the creeps exactly or they won't die.
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When you get near the base build a zigzag one one side, cover the base so they can only get in the opposite side. The key here is to upgrade the towers again along the flight paths, so you have an inverted V of upgraded towers. Just get the lasers asap again, it gets tricky as you have to keep upgrading or you lose. Even after the lasers, if you don't keep upgrading along the flightpath you lose.