I love this style of game a lot, looking forward to the full version.
I feel like there are some potential balance issues, depending on how difficult you want the game to be and the total length. You can beat the demo if you max out all the tree (only one I didn't max out yet was tower health), which then poses an issue for the following levels if enemy strength keeps going up without enough damage output to match. I have a few suggestions (take these with a grain of salt if it doesn't match the difficulty you're shooting for):
1) Beef up the cursor big time. Two good options are by increasing the radius or upping attack power. Because of the spacing of enemies, typically you are only hitting 1-2 enemies at once. Also, the ever-increasing health means you can't 1 hit KO anything (except on crit). Increased radius let's mouse be your targeted crowd control weapon. Alternately, upping damage and/or speed would let mouse be a targeted piercing attack (like the rogue).
2) Earlier access to AOE type attacks/change order of upgrade unlocks. The first few attack upgrades: bow and freeze spell are both effectively single target. Lightning would be a far more effective early upgrade because it can chain eliminate many small enemies in the early waves which is what you face. Cursor freeze is only really useful in later waves when you have more heavy hitting boss monsters to target.
3) Queens blessing is almost too OP, it's the only one that matters. Because bow and lightning are the only 2 effective attacks, as long as you get a good arrow buff you can win. Only problem is that when you lose it the next round you are then severely under powered until you get the good bow upgrade again. I like the random element the queens blessing provides, but the balance is off so instead of testing out different strategies for success you basically are just level grinding until you the right blessing. Hopefully in the full game there's enough level variety to warrant buffing each weapon type.
That's what I've got for now. Love the art style! enemy design is fun too!