I played your game, and here are my thoughts on it:
-I think there's a solid idea here. One suggested change I would have is toggling an option to have units either automatically target towers or proceed to the goal.
-Another idea would be to have additional ways to spend upgrade points in between stages in other ways than unlocking new units. I had some extra points that I couldn't use in between matches after unlocking new unit types. I also saw that it said I had 0 upgrade points during battles - is this a bug or are they supposed to be different kinds of points?
-Hotkeys would be nice to have, such as for using the healing spell. I also wanted to be able to disable the music and sound effects while playing.
-The game runs kind of slow for me, but I'm also playing it on a laptop I got in 2017 so it's probably not something to be too concerned about. Still, having the option to reduce graphics settings (e.g no shadows) would be good.
-The dark sword icon against the green Available Units window is really hard to see - it needs to be brighter. Same with other unit types.
-There's a lot of potential to introduce more counterplay between player units and the towers. For example, a special unit that automatically attacks towers adjacent to your units, or one that powers up your units' stats while attacking towers.
-It's not clear how your score is calculated at the end of each match.
-A way to make the game more fun would be to add additional kinds of challenges for various stages. For example, win a match on wave 2, or win a match without ever healing your units.
-I think you should be able to select groups by double-clicking on one of the units. It was annoying having to use the drag box on units that were actively moving.
-I liked how each stage introduced a new type of challenge to the player. One recommendation I have would be to visually distinguish towers that are out of reach for melee units as opposed to having them look the same as the others. This also applies for any other type of tower.
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The game is obviously very early in development, but there's definitely a good idea here! I didn't see much of an issue with the art given that the game is in the prototype stage, but I assume it will be refined further into development.