After playing Zelda 2, I was reminded of this game while playing it again recently. I feel like both Zelda 2 and Castlevania 2 have a lot of similarities, especially for the town scenes and the poor translations.
To this day, I still think about Castlevania II, as it was the only game I had as a kid. Even if I don't like it, mechanically specifically, too much myself, I still adore it and play it a lot. And I still wonder what might it have been like had a game been designed in a way similar to Castlevania II but with better controls, map design, and implementation of mechanics. A pseudo open world adventure game, with some RPG Mechanics and a Time limit all sound like they COULD come together very cleanly in a mechanically nice way to make for a very solid Gameplay loop. Especially if the game itself is intended to be beatable in only 45 minutes or so. Thus making it more about repeat playthroughs where the variety comes. Maybe if I ever finish up some other works I have, I'll get to programming a Gameboy game like that. I think it could be fun. Also I was surprised you didn't mention the broken ending stuff. If I recall the ending texts are not correct for the image they are associated with, but I may be misremembering.
This game i tried a really long time ago but i couldn't get past the first castle as i just fell through the floor and then just gave up. I then just got rid of the cartridge.
