Nice game! And beautiful music, like other's have mentioned. Easily my favorite part of this game.
I'm not really used to playing games of this genre, so forgive me for my lack of constructive feedback. The game overall reminded me of Fate, an old dungeon crawler, especially with the use of large rooms with enemies, upgrades, and bosses, all of which are well implemented in this demo.
As for improvements, like others have mentioned, I think the game could benefit greatly with character animations, especially as the game's thumbnail shows. The bosses also look interesting, especially the one shown in the thumbnail and the tree-boss seen in game. What other bosses are planned, if I may ask?
Overall, very solid foundation. Best of luck with development!