I had so much fun playing this! I was really bad at it in the beginning but the difficulty curve is perfect and therefore every time I got a bit further I always felt that I can do a bit better the next time and I wanted to go back in. I eventually had a run where I picked the perfect upgrades for my defensive playing style and I got to the last level and only had three drones to go... and then I got cocky and let my guard down and... well... there was some swearing.
I would suggest "game over" text instead of "restart" when you die, and/or some kind of juicier indication of the area popping up on screen when you get to a new area/start a new game, because for the first few games I thought I was just restarting the level I was on and it really confused me then when the upgrade system kept changing; consequently I didn't understand the system at all for the first few games. Once I understood it I really enjoyed seeing what the different choices would result in.
I think the fact that it's procedurally generated is crucial to what makes this fun to play over and over. I also like the fact that there are subtle changes to the play area as you progress, including to the palette.
I like the visual design of the upgrade system but I think the player HUD could use some love if you want to make a polish build. (I actually think this game would be good as a "training" system for a larger game.)
It's a nice touch that the drones have to rotate to face you and that they can be tipped over and will roll around. I had one in a game that just would not die as I kept unintentionally just clipping it and it ended up rolling all over the place, which was really funny. Having read your explanations about the environmental damage, which I began to suspect was the case later in the game, I presume it just wasn't close enough to anything to break it for a while.
I love the choice of music.
Getting distance shots and trick shots right is incredibly satisfying.
Some small notes:
Typo "Cick, hold, release to fire".
Why not call them arenas rather than areas?
Please remember us arrow key players and that we need a jump key nearby our right hand.
The music in the area prep spot/staging area/behind the big wall that plays before you start an area doesn't loop - possibly also the music that plays when you're playing.
With Strider you can go into the area you've just finished and fill up your health bar before you go to the next area. I don't know if that's an intended perk or me taking advantage of a loophole, or if it doesn't really matter because you can use Strider to regain health anyway when you start the next area and you're in the staging area. That area was really necessary for me as a defensive player in the later stages of the game (Strider or no Strider) as I wouldn't have survived five seconds running straight into the mayhem, as I'm sure some gamers do and are good at, so I have to disagree with Ben about making it go away. I would hate this game if I was always pushed forwards. (I already just learnt to let the disappearing gold go later in the game, which was frustrating but was the tradeoff that I realised I would have to make to stay alive and so I had to make gold using upgrades instead.) So that's your challenge if you work on this game further - how to balance completely opposite play styles.
On the whole, though, well done!