I like the idea, but this game really needs to have some feedback. Half the time, I have no idea what my dice effect did. I know it's not always helpful, but it wasn't clear to me if I didn't throw my die in a good place for the effect, or if anything even happened.
The enemies also seem to be really really blind, they never noticed me.
I think this D20 idea could work well in a different kind of game, maybe like a wave shooter, rather than a preset level.