I managed to reach the end of the demo in my first run without reading the tutorial and without dying, I must have been lucky! But it tells one thing: even if it seems obscure at first, you do get the grasp of how things work pretty quickly! Ended up doing three full runs since your progress is saved you keep accumulating power, which is nice.
I really like the retro GB graphics and music (of course more tracks depending on the places/situations would always benefit the game). UI is simple and effective.
As some have already noted, the arrows to the Sanctuary and Temple are reversed in the second area.
All in all, I enjoyed the game and developing further on it with more startegies, areas and enemies will make this game even more fun and interesting!