In my opinion, Ghosts 'n Goblins hit the arcades like a copernican revolution in 1985. It's one of the undisputed strongest early Capcom titles and, compared to their library at the time, it feels very modern. It has distinct levels (for the most part), actual bosses (if recycled) and a difficulty that is ALMOST manageable for the majority of the game. It certainly is more playable than Tatakai no Banka, for example.
That said, I was confident in my experience with the infamous NES version, considered one of the hardest games ever, that I have succeeded in clearing with a single credit. Well, let me tell you, arcade G'nG will stare you down and whisper "The kid gloves are off" before punching you in the nuts. You probably know the game is a sidescrolling action platformer (one could say it's a run and gun really) with a slightly cartoonish horror theme. You control the knight Arthur and must rescue the princess, kidnapped by the forces of evil. You get a variety of weapons to act as your projectiles and must face a tsunami of ghouls, goblins and even some ghosts here and there. The levels are distinct: a graveyard, an abandoned city, a lava filled cave and so on. Most of them are divided into two sections with one checkpoint. Mercyfully, you can continue indefinitely as long as you pump quarters. The game is very hard, much harder than the dreaded home console port, mostly because enemies and projectiles are plentiful and controls aren't very reactive. Stairs and even firing kind of lock you in place, jumping has a fixed arc, so when certain patterns overlap or stuff gets thrown your way quickly it's hard, sometimes impossible to react. That said, the game is actually plenty playable with no emulation assists (mostly), even if hard to deal with (especially the iconic Red Arremers). The last two levels definitely go overboard, though. There are multiple random factors involved, often overwhelming you, and the boss of those stages (Satan or the extra large Red Arremer) is almost impossible to deal with, because it swoops down like the Arremers but has much more health (obviously) and an elephant hitbox that's extra tough to avoid, especially with the cramped spaces of the last level.
The presentation is absolutely superb. The whimsical horror theme is absolutely iconic, as are a lot of enemy designs. Arthur famously gets down to his boxers when hit and the whole game has this touch of fun to slightly take the edge off the brutal difficulty. Levels look surprisingly unique for the time, give or take the last two levels. Even the slight variety of bosses is commendable and a step forwards in a more modern direction.
Music is legendary and quite special for me. I hum these melodies all the time and love them dearly. They sound a bit rough with the arcade soundfont, but they're classic tunes nonetheless.
Despite being hard as all hell and sometimes unfair, Ghosts 'n Goblins is a cornerstone of early Capcom' s library and a game that was way ahead of its peers (and even of some later arcade games). Essential when you want to experience some pain! I sure do every once in a while, even if the sequels are superior.