This has potential, but is in way too much of an unfinished state for me to comment on the gameplay. Good to know that you will make chests mean things, and Atk/Def matter. Maybe they could have weapons (+ATK), shields (+DEF), gold (experience) and potions (HP)? Make them need keys to open, so they're a bit of a risk/reward mechanic?
You nailed the GB aesthetic, I love the simplified monster, knight, item and door sprites. Maybe have later levels use more and different music tracks?