Very nice game, I would love to see harder levels.
Not necessarily levels that require more patient or timing but thought.
There's nothing like overcoming a really mind bending puzzle.
I like how every level has an elegant solution where's level 20 is just brute force ^^.
advices/tricks for finishing everything on CRAZY:
1. No tower or building should be ignored, they all have their purposes and none are a "waste of minrals/energy"
2. unimporved buildings give you your entire money's worth when selling, this is a HUGE help,
you can recycle towers when you're being attacked from different directions, for levels when you need
a lot of energy for a long time you can buy lot's of energy stores (unupgraded), and when they run out of juice sell them and buy more solar stations,
3. this is a fun trick for mining levels, you can delay enemy ships with energy relays instead of actually killing them (yes build relays where enemies come from to keep them busy), it's low-cost and should allow you to spend big bucks on mining, (keep in mind, this defence kills suiciders)
4. in levels where there's something moving that needs constant defense, best way to go is lot's of unupgraded lasers where-ever the thung goes, and THELs against mother ships
Hope this helps.
btw a little tip, in levels where the enemy isnt targeting a specific target, but your entire base, you can easily delay mother ships by building lots of energy relays, they will keep attacking the cheap relays and not make any damage to your base, while you kill them with rockets.
i'm not sure how but the 300 seconds challenge isn't necessary, if you complete everything in 240 seconds or less the game will end and you'll receive the badge
 "Electric Box 2"){kind=icon}
 "Gravitee Wars"){kind=icon}
 "The Company of Myself"){kind=icon}
 "The Space Game: Missions"){kind=icon}
 "Toss the Turtle"){kind=icon}
 "Learn to Fly"){kind=icon}
 "Achievement Unlocked"){kind=icon}