Initially gave a 5/5. After beating the game 3/5. If you want the 5/5 back you'll need to fix the lag. Fix the Key/Chest screen. It by itself is losing you a star. It's sluggish, pointless, and empty. My suggestion would be remove the key from the level and make it a locked costume piece. Fill out all of the item sets. Tweak the difficulty and jump timing.
Need options for time spent vs possible rewards. Say let me choose to spend 4 years studying. Reward would be more money for working. Training could increase various stats. An option to complete quests for gold, information, and/or items would be nice.
I would also like to point out that initially I gave your game a 4 star rating. After purchasing every upgrade, every weapon, every cannon and playing for several hours on this game I have reduced that rating to a 1 star. Your game has very simple mechanics, make it easy to go far make it harder to go further. If it's about altitude you might want to might your altimeter display more info.
Love that I can get more vertical speed than horizontal, rocket boots might as well be left out. Highest possible horizontal speed but next to 0% chance of getting there.
Really simple. Not a lot of depth to this game but very fun for what is there. Maybe some extra modes or let players pick their own objects to hurl at the planet.
To easily beat the game.
1. Start with 2 Pierce towers closest to your toy pile.
2. Get enough souls to buy Slow and DoT towers next to Demon Spawn 3 High
3. 2 more Pierce towers on top of first 2 Pierce towers
4. Extra Range Damage tower on top of 4 Pierce towers
5 . Change Direction towers facing Pierce towers
 "StrikeForce Kitty"){kind=icon}
 "Notebook Space Wars 2"){kind=icon}
 "When I Grow Up I Want To Be A Dragonslayer (mobile)"){kind=icon}
 "Air Heroes"){kind=icon}
 "Crush Kill Destroy!"){kind=icon}
 "A Nice Day"){kind=icon}
 "Flick a Boom"){kind=icon}
 "Canoniac Launcher"){kind=icon}
 "From Beyond"){kind=icon}
 "Skyloon"){kind=icon}
 "Demons Took My Daughter"){kind=icon}
 "ef the game"){kind=icon}