Bomb Jack was a great game, this seems to ruin that though by the random spawning of enemies. They appear underneath you and make an otherwise good game "unfair" as many others have commented.
Even more strategies:
Also, its essential to keep in mind that different turrets are useful at different times. Fire turrets are better at high levels, where worms live longer. Gatling guns and rocket towers work well at the start when you have few towers and limited strategic options, but later on, other turrets work better. Just going for one kind of turret isn't going to work: freeze towers etc. are no good at the start, you need other guns first to benefit from the freezers. It helps to line guns up with the trees, so worms run straight through the guns with the apples. There are 3 strategies for dealing with towers being destroyed: 1) kill the worms quicker, which involves using a better combination of turrets for the level you're at. 2) use cannon fodder, weak turrets like handgun circling your better turrets. 3) lots of upgraded repair towers. The game requires an evolving strategy that changes as the game progresses.
More strategies: blizzard and the freeze towers slow the enemies down, giving you more time to kill the enemies using your other guns. They do nothing on their own. The flame thrower (from the fire catapult) can easily kill worms when placed in line with your apple trees and backed up by something that freezes, but you need to be able to survive to get to that point and as you've found rockets and some of the other turrets are good for that. The fireball towers do shoot randomly, but can be quite effective once you have lots of them, and they are relatively cheap to build. Also, damage depends on more than just the number - some guns have explosive damage and hurt multiple worms. Others like fire travel through worms and hit them multiple times.
kftaurus, thorn thrower and a few other turrets do not target enemies and instead fire in all directions. With thorn thrower, the worms are damaged when they walk over the thorns.
Draco18s, it seems like you've played the game quite a bit. But you may have missed some of the crucial upgrade paths and some of the most powerful turrets. Try the turrets like blizzard and flame thrower that come after these upgrade paths: Snow Launcher > Thorn Launcher and Catapult > Fire Catapult. Also towers that freeze the worms help a lot as all your other towers can do more damage.
Flostin, great effort! The tree stuck in dying state bug was fixed 12 hours ago. Are you still causing it to happen? If so, could you please explain how you cause it, so I can get the developer to look at it again. Also, you can get to level 51 and possibly even higher without using any bugs, although I would call it a clever trick, I won't say more as I don't want to give the answer away.
For those finding the game hard, its designed to challenge you to think up some different strategies to try out, there are many possibilities. If you're in a hurry, try reading the new hints / tips / strategies section I added at the end of the game instructions - hopefully that will help.
MordredofFairy, thanks for the comments, you have some great ideas. But like you suggest, Apple Defender is trying to be different to the standard tower defense games, which have probably started to get boring by now. You can successfully build quite a few trees and keep them alive, the trick is their placement - try to build them far over to the right and in a straight line towards your turrets, this gives you maximum time to kill the worms once they steal the apples and forces them to walk over your turrets.
I'm unsure where you found "start game". The game loads to the title screen without needing any interaction and you just left click on "new game" in the top right to play.
 "Bomb Jack"){kind=icon}
 "Whiteboard Tower Defense"){kind=icon}
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 "Apple Defender"){kind=icon}
 "Shape Em Up"){kind=icon}