I feel like most of the challenge came from the controls either being over- or under-responsive. Nothing really made me stop and think about how to solve the level.
Can you put in a high contrast colour scheme? I'm partially colour blind, and it's really hard to tell the difference between the two squares, and between the goal/possible-walls.
I like the greyed out idea. I'd also like to see a list of everything i can create at the moment, given my current elements. Mean's I'd spend more time thinking about how to make X rather than blindly trying combinations.
Add either wasd support, or let the whole thing get played with the keyboard. Swapping my right hand between keyboard and mouse constantly was frustrating. Otherwise it was a good game. 4/5
Here's the trick. You only need 1 or two collectors, upgrade them as much as you can quickly. IF the collectors aren't in any danger, just put one big defender on your base, so he covers the entire tree. Otherwise one defender on each attack path. Every time a defender is about to die, upgrade him. It's only 10 coins more, and it adds an extra shield for next time. Don't repair until they're maxed out. That's basically all there is to it.
two things it needs. 1) it needs to snap movement to set distances, so you don't have imperceptibly small collisions stop movement. 2) it needs a wireframe view option, so you can see all the interior details.
The events are very annoying. Get rid of them. They remove so much of the strategy. Also recruiter limit is too low. I was needing more then 10 new men most rounds.
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 "Quadrilateral Thinking")
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 "Achievement Unlocked 3")
 "Days of Monsters")
 "Space Arcade (the game!)")
 "Doodle God 2")
 "Bullet Heaven")
 "Steelcurse")
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 "Not Your War"){kind=icon}
 "Critter Forge")
 "My Pet Protector 2")
 "Death vs. Monstars")
 "Interlocked")
 "Burrito Bison")
 "de Blob 2 Revolution")
 "Spitfire: 1940")
 "Adapt or Die")
Looks like Thorum made good his threat. It was a great 3 years for you though. Cheers!