Great twist on a classic mode. This one has a lot of potential. I could definitely see something like this sitting next to a game like Forager on Steam. The visuals are nice and clean without being too simple. A bit of polish would really make it pop.
In terms of gameplay, the concept is great. I would probably make the level curve a bit smoother. You could also add new ways of obtaining crystals (e.g. enemies which drop crystals but only for a short period of time) and crystals of greater value. I would also like to see a second progression system which extends beyond each stage. One would reset between maps and confer larger bonuses while the other would grant smaller bonuses but would carry between maps. An alternate game mode or optional challenge could turn something like this off.
Finally, the boss fight is a neat concept but it would be cool the boss itself could move around. To that end, enemies (and bosses) should be able to attack the player, damaging your map health. Optionally, giving the player a form of attack separate from the tower would add a lot of depth while keeping the "single tower" concept intact.
I've played a lot of these games - I like what i'm seeing here. Hopefully this feedback reaches you. Ultimately, it's your game and your vision and it's better to nail the core loop than risk feature creep.
This demo shows a strong foundation and I would love to buy the final product.