Made it to level 30. I have some suggestions:
Tutorial: Quality-of-life enchancement.
Ability to remove/switch out pegs: I really think this should be a thing, to allow players to change up their layout mid-run. Placing a peg on accident or wanting to perfect a certain peg layout hurts when you are unable to remove an orb. For example, I wanted to see what the eletric peg would do but now I can't really, and I'd like to replace it with a turret but cannot.
A fun, related idea would be to have the ability to upgrade a peg to a random, more expensive peg, because currently there's not much to use the money on (Also, upgrading the freeze peg doesn't seem to upgrade how long orbs are frozen for, which is what I thought would happen).
Ways to make the game more difficult when going to higher level: Currently, it seems the only difficulty increase with higher levels is more orbs, and maybe ones that take more damage to break. This makes levels take longer and ultimately become kind of grindy/boring.
So, every 5 levels or so, a "curse" could be added, something like "each orb has a 1% chance to duplicate itself when breaking into a smaller orb", or "each orb takes 1 less damage each time it takes damage", or "5% of orbs pass through the first peg they encounter", or "increased gravity: all orbs fall faster". This would make the game more and more difficult at higher levels.
Alternatively, making the basic health purchase go up every few levels (for example, every 5 levels it costs 5 more coins) would also introduce some progressive difficulty. It feels like it is more cost effective to just keep saving money to heal, especially when upgraded pegs don't seem that much more powerful.
Balancing: Clearly, the freeze peg is insanely good. I like the mechanics of it currently, but maybe make a limit on the number of each type of peg is available. This would encourage variety in layouts as well, making it so (what I believe is) the extremely powerful combo of freeze+fire+turrets isn't as good.
Bug(?): When orbs get frozen and then drop to the flat areas on the side, they lose all their momentum and usually stay on those ledges. I thought having turrets would solve that problem, but often when the turrets shoot the orbs they go towards the center for some reason, launching themselves into the holes. Probably caused by some interaction of the bullets and the orb when they colldie? I find it really annoying, as I lose out on potential money - when the turrets shoot the orb, I would expect the orb to move towards the corners, where it can just be shot repeatedly to make a profit.
Anyways, this game has a lot of potential, hopefully my current build is nerfed a bit, the usage of other pegs (and upgrading pegs to higher levels) is buffed, and maybe some sort of gradual difficulty increase is added.
Aiming for level 50 or bust!