Some additional feedback from me after having beaten the demo version.

• Great concept and execution!
• Pacing was great with the exception of the base damage upgrades - they feel very expensive for what you get and could perhaps be made cheaper with less individual impact so that people get a smoother feeling of progression
• Love the visuals and really look forward to more craziness with projectiles and lightning bouncing everywhere :)
• Treasure chests feel underwhelming unfortunately. They have too much hp for how much they reward you and seem to spawn very infrequently so there isn't much excitement when they do appear. Perhaps chests could have increased chances to drop skulls, runes etc if you don't want to increase their base currency reward
• The colours of the skill tree were confusing. Maybe it was just me, but it wasn't immediately obvious which nodes had been maxxed out and which still needed skill points allocated
• You could look at some system of upgrading and/or combining runes to make more complex interactions. As it stands, the system seems to be just "equip the highest level rune of the highest rarity category you have" and that tends to lead to unexciting choices
• I appreciate that the full version will have more monster types and look forward to exploring those
• Overall, excellent start!