Craaaawling in my skiiiin!
This elf he will not heeeeal!
So, Treasure made an action-RPG in the Zelda style, eh?
Ayup. It's pretty comparable to Landstalker, which we've looked at here previously. Has the same isometric perspective and sword-swinging, block-pushing, trap-dodging, puzzle-solving action.
There's also perhaps a little The Immortal influence here, in that it's basically one long dungeon crawl that has you going to deeper and deeper levels of a dungeon based in this one town that you just continually revisit. To compare it to a game that came after it, the structure is very much like Diablo ... which is probably a better comparison, since Immortal didn't have a town ... it does really have Immortal's vibe though, with all the dungeon deathtraps and the gratuitous decapitations and barbequing of the enemies.
Treasure has an arcade aesthetic to pretty much all their games, and don't expect it to be any different just because it looks kind of Zelda-ey. The plot is very threadbare, some typical "save the kingdom from Dark Phallus" generic story and the tiny handful of people in town only give you cues as to what to do next, or spew out some generic nonsense dialogue.
Two games that this game probably influenced that it never gets credit for - Diablo, as you mentioned, and Castlevania : Symphony of the Night (due to the gothy health refill and save stations in the midst of the dungeon floors, as well as the teleporters that let you cycle through them as they are discovered).
It starts off pretty fun, but give the game about two hours or so and it will start to grate on your nerves. Like most Treasure games, it's full of a lot of cheap bullshit and tedious sequences. And the play control is definitely on the loose and slidey side - this ain't Zelda 3.
The graphics are OK, and there are some impressive boss battles. The music is likewise pretty good, a mix of moody ambient pieces and some high-energy stuff that uses the Genesis sound chip pretty well. Has a fair amount of voice clips as well.
The play control is decent most of the time, but it's little tics will end up getting to you in the later difficult sequences. First of all, the isometric perspective makes precision jumping very difficult (as anyone who has played well into Landstalker will undoubtedly attest). The collision detection on your sword swing is also weird, and hitting enemies can be a pain in the ass, especially the ones that are low to the ground like the common slimes that you face.
Generally speaking, the battles really aren't smooth and that's probably the biggest failing of the game. It does have an interesting magic system though, reminiscent of Gunstar Heroes. You stock up the four element types, and then can use them in different combinations to create different spells.
It's an interesting little experiment, and has it's qualities to be sure. It's pretty uneven though, and just gets to be too annoying after awhile. Action-RPG fans might want to take the gamble on it though, and see if it's to their tastes.