Use Spacebar to shoot a fireball. All enemies have 1 hp so it should be a 1 hit death. If you kill them outside the rune circle they'll drop a soul, which you can use to purchase upgrades...I didn't have enough time to add a tutorial level sorry about that. I've updated the game description with the controls.
Sorry for the late reply, it took me longer than I'd like to admit :'( It should correctly scale to your screen size now. If it still doesn't work please let me know I'll release the binaries asap.
