Very pretty game with some nice audio. Like others, I had an issue with sound not working on every load, but that's presumably a Godot bug of some sort.
Gameplay wise, I wasn't really clear on what to do until I read through some other comments, but once I figured it out I enjoyed it. it's an interesting idea with the drawing mechanic for casting spells and think there's a lot of ways you could expand on this idea, though I did find holding RMB to draw so often a little uncomfortable, though I also use a funky mouse so that may be on me :D