I have to start off with kudos for setting up tutorial and quite satisfying game :) Although I feel its a very early, rough at corners prototype. If you continue polishing it you will probably end up with pretty decent game and maybe even come up with more unique twists.
Here are some advices/comments on what I saw:
1. It was unpleasant surprise that mouse scroll did "scroll" the tutorial. Was expecting only mouse click. If I'm not mistaking this can be easily fixed with mouse filtering or by listening certain events (like mouse click)
2. Attacks felt unresponsive to me. Maybe animation or initial speed could be better. Had a feeling that everything in game is moving slow and less responsive as it should
3. I think you picked up wrong scaling option in Project settings :) The menu and other UI elements feel like they are just stretched with quality loss. Try looking into it and fix.
4. Wasn't a fan of instant actions (Lvl up/Game over). Would be nice to have at least a minimal pause between what happened & result. Like gradually slowing the game until full pause and then showing the upgrades (ideally with SFX) and then gradually resume the game. Just an idea.
Hope these comments will be helpful to you and nevertheless you did a great work and learned many things.