dude the movement is SO SMOOTH. Building up to a full moveset in metroidvania fashion would be an easy way to implement some progression, as would attack strength / quiver size / arrow effects (light/fire arrows?) and max health pickups. even just a few palette-swapped enemies with altered speed, power, health, and/or movement patterns would add a lot of diversity as well as setting of 'harder' sections of the castle. I don't know what all you've got planned for this, but what you've got so far is looking pretty effin' hot!