Nicely done, the atmosphere and game design as a whole felt like a NES title of it's own
(Ah, now that others have mentioned it, yes, this reminds me of Lolo's Adventure)
Only some cosmetic issues, other than that I found no bug:
In stage 2-8 (Introduction of magnets), the hint sign's last line
"I also works through.."
should be
"It also works through..."
Also, the image at the right is wrong, it shows red magnet repelling while blue magnet attracting; the image at the left is correct. red attracts, blue repels.
I think you can add the sprites that shows spikes protruding from the ground for the eye monster. Right now you just go "puff" and I feel it's a bit lazy. Or you can just go the other way and change the spike to a mouth/black hole to match this animation.
Lastly, at times (I think it's mostly in stages with artifacts), when you get the artifact and the medal,
there seem to be a sort of sfx outburst, and I had to constantly tune up/down the volume.
Hope to see some back story if there's a sequel! 4 stars.