8 Eyes is a rather blatant Castlevania clone, with a dash of Mega Man in the mix, as you select from eight different levels at the outset that each end with you getting a new type of sword, and each level's master is weak to one particular sword type. It does, however, add a few ideas and qualities that are interesting. Unfortunately, the gameplay is executed so poorly that those ideas are rendered irrelevant.
It's your standard "awkward angles" action-platformer of the period; you stab straight forward, but the enemies consist of either guys that have a longer reach than you, or flying enemies that swoop in at angles you can't really stab at effectively, so you have to do a lot of finicky jumping and wriggling to hit anything. And of course, true to '80s style, everyone takes multiple blows and just tanks right through your strikes like they're on PCP, making it extra awkward and annoying. Now, the NES Castlevania games did this too, but they got away with it thanks to thoughtful level design that made the awkward action manageable. 8 Eyes' level design, on the other hand, is much more amateurish and slapdash, with tons of cheap hits that are virtually unavoidable due to positioning.
One unique twist is that you have control of a falcon that perches on your shoulder - pushing up + A sends him flying about on his own, then you can command him to swoop and attack by pressing down + A. Unfortunately, aside from being as clumsy to control as the main character, the falcon also has his own life bar, and is just as easily thrashed up by the enemies as you are, making him only minimally useful.
One other issue with the game is just plain obtuseness. There's no clue whatsover as to what order you're supposed to tackle the levels in initially; you find clues along the way, but only by attacking completely random wall segments with the falcon. WTF? Then, apparently after you beat all the level masters, you have to go back through all the levels in a very specific order, and if you complete one out of sequence it ends the whole game on you. Again, WTF.
The game's graphics are typical murky blah NESness of the late '80s, but it surprisingly has an epic soundtrack with like three pretty good musical pieces per level. Shame that it was wasted on a game with such awful gameplay and structure that most players won't ever hear 90% of it.