The clear comparison for Getsu Fuuma is Zelda II - you travel around on an overhead map, enter side-scrolling dungeon areas, occasionally get attacked in the field by roving monsters wherein you enter a side-view battle against a motley crew of them, etc. There are enough little unique wrinkles for it to carve out its own ground, however. Also takes place in Japanese Hell. Let's see ol' elf boy handle that one.
Metal as F
Zelda II very well could have been an influence here, as it came out in Japan six months before this one did. Fuuma was likely already in development before it came out, however, and you see all sorts of influences from other Konami franchises at work here. The biggest single influence is easily Konami's own Goemon; it borrows the occasional first-person dungeons (also used in Goonies II), and though the first NES Goemon game didn't have side-scrolling platformer levels, those feel remarkably like the arcade Goemon
(complete with the traditional-ish Japanese music and the slightly slippery feel).
The overhead map is also clearly the base for what would go on to be re-used in Teenage Mutant Ninja Turtles and The Lone Ranger. And the first-person dungeon areas are a little more advanced than those seen in Goemon or Goonies II. They actually appear to be a retooled version of the first-person "in the base" levels of Contra!
Anyway, the setup here is pretty simple. The forces of Hell are apparently intent on escaping the netherworld and invading Japan. Our main dude (Getsu?)'s brothers went in to put a stop to it, but died and lost their special family swords. So now Getsu heads in to recover the swords and take out the Main Demon himself. Of course, the Main Demon is conveniently located behind a maze of twisty passages and a sizable army of underlings.
You start out in the overworld, which is non-linear. The many (but fairly brief) dungeons are usually just a mandatory obstacle to forward progress, but some contain necessary items, while others hold nice bonuses like a health potion or an extra life. The only other thing on the map is the occasional hut, which usually houses a crone or monster of some sort that dispenses not-very-helpful advice, but once in a while you'll run into a shop in these.
The game consists of a series of four islands that have to be tackled in order. On each island, the basic goal is to find the pass being guarded by a boss monster, which lets you get past a gate guard (who I have no idea why we can't simply kill) to get to the next island. Each island is a big sprawling mess, however, with scads of small dungeon areas. You'll also have to find some other items in these dungeons to progress; for example, the first major goal of the game is to get to the lone shop that sells the Rock Breaker sword, which you need to clear some passages that prevent you from leaving the first island. Though it's not strictly necessary, you'll also want to pick up the War Drum in one of the early dungeon areas, an equippable item that lets you shoot a beam a short distance and makes certain enemy types immensely easier to kill.
There's a 100% English fan translation that has been around since 2000, but I was never able to actually apply it to a ROM and get it to work. I ended up playing the game in Japanese, which is far from a deal-breaker unless you're absolutely unwilling to look anything up online. If you're disinclined to use outside materials, expect to do a LOT of aimless wandering, as the game really gives you no indication as to where the next item you need is located.
There's a couple of hinks to the gameplay you should know about, even if you don't want outside help. First, you can jump at any point after sliding off a ledge. I don't think you ever need this mechanic to actually clear a pit, but it often helps immensely in avoiding environmental damage from ground traps and hanging tentacle things. The other thing to know is that to use items in your inventory, you pause the game with Start (while in a side-scrolling level) then use A and B to cycle items in the two available slots.
Getsu Fuuma is a very decent little action-adventure, though there's a few things that grate after awhile. One is simply being lost most of the time and having to wander about hoping to stumble on the next important item or area. There's also the somewhat slidey and loose gameplay; enemies are the usual Suicide Drones who just charge you hoping to do chip damage before dying and tank multiple hits. It would get overwhelming in the later reaches if not for the fact that many areas have a point where you can simply kill endlessly respawning weak enemies over and over to charge your health back up (health drops are extremely frequent).
The first-person areas aren't fantastic either. They're a little more advanced than previous Konami levels of this nature; you see your sprite over-the-shoulder, can fight enemies and can even jump while attacking. If combing through them trying to figure out where you were going in Goonies II gave you a headache, however, get ready for that same headache here. There's no map to speak of, just a compass that gives you a general idea of where the exits are. Combat in these areas is also clumsy and rudimentary, more something to hack through than enjoy.
It's thus more of a curiosity piece for those interested in old-school Konami (or old-school Metroidvania-ish games), but it's still pretty solid on its own merits and packed with a lot of neat little graphical flourishes. If you liked the atmosphere of Sweet Home
you might also dig this, though they're two totally different styles of game.