Baleful Polymorph: A magician who likes to hide in treasure chests does this if you don't kill him fast enough. Being hit by his magic turns you into MANY different things depending on game/armor status, including a girl, duck and seal that all can't attack, and an old man that can...but slowly. Another transformation is a bee whose "flying" hitbox can actually be beneficial and allow you to walk across lava.
Blade on a Stick: Arthur's default weapon is a Lance, which the Japanese version originally called a "yari" (spear).
Blocking Stops All Damage: Ultimate Ghosts 'n Goblins introduces shields which will unfortunately break after a number of hits. It is possible to obtain an unbreakable shield from a witch though, after which anything block-able can be stopped with impunity.
Boss Remix: The boss themes in Ghosts 'n Goblins and Super Ghouls 'n Ghosts are remixes of the themes of the levels leading up to them (although the boss remixes for Stages 3 and 5 of Super Ghouls 'n Ghosts use the level musics from Stages 5 and 3, respectively).
Boss Rush: In the sixth stage of Ghosts 'n Goblins, the bosses from the previous stages show up again.
Bottomless Pits: Not nearly as common as in most other platformers that use them, but they're definitely there.
Bowdlerise: Lucifer and Samael, the final bosses in Ghouls 'n Ghosts and Super Ghouls 'n Ghosts respectively, were renamed Loki and Sardius respectively (except in the arcade version of the former, where he was still called Lucifer).
Cap: In the original game, there can be no more than three zombies on the screen at a time. Other enemies have similar caps.
Check Point: Only works on death or when you insert 25 cents within 10 seconds.
Clean Dub Name: In the console ports of Ghouls 'n Ghosts, Lucifer was renamed Loki in the localized versions, while Samael became Sardius in Super Ghouls 'n Ghosts.
Clown-Car Grave: The zombies just keep... on... coming. Forever. And they're not the only ones.
Crazy-Prepared: According to the ending of Ghouls 'n Ghosts, Arthur had spent three years improving his weapons because he knew the demon world would be restored eventually. Not that it did him much good...
Difficulty By Region: The American release has the mercy of Meaningless Lives, but the Japanese version has no visible continue option (you have to use controller commands when pressing start).
Double Jump: Arthur gets the ability to do this in Super Ghouls 'N Ghosts. It does not make the game any easier.
Dual Boss: In the first game, the bosses of the second stage are twin versions of the cyclops from the first stage. Likewise, the boss of the sixth stage are twin versions of Satan, the flying devil boss from the fifth stage.
Dub Name Change: The official name of Arthur's beloved is "Princess Prin-Prin" (as displayed in the ending of Ghouls 'n Ghosts), although some of the earlier localizations renamed her into "Princess Tamara" (in the Genesis version) or "Princess Guinevere" (in the SNES version, a reference to King Arthur's own beloved). However, most official documentation (even for the Japanese releases) don't even bother mentioning her name at all and simply refer to her as "The Princess".
Expy: Knight Arthur is technically not King Arthur, but just some knight who happens to be named Arthur. And the princess he's saving is officially called "Prin-Prin", but has undergone a few name changes between different localizations.
Fake Longevity: You'll have to play through the game twice to see the true ending. This applies to every installment of the series.
Losing a life to getting back into the game: 13 seconds. Getting a game over to getting back into the game: 24 seconds. Considering how hard the game is, this adds a LOT of playtime to the game and make it seem much longer than it actually is.
Item Amplifier: In the sequel Super Ghouls and Ghosts, picking up turquoise and golden armor gives your weapons more power and in the latter, the ability to charge up your attacks.
Jump Physics: Present but not highly distinguishable. Jumping from foothold to precarious foothold isn't as big a part of gameplay in Ghosts 'n Goblins as it is in someotherplatformers, and Arthur's jump isn't all that much more controllable than a real-life jump.
Commodore 64 wouldn't allow jumping over a tombstone if you were right against it; you needed a single pace back before jumping over. The "good" versions don't have this quirk.
Power-Up Letdown: The torch and the axe in the first game. Made more insidious in some versions because the Power-Ups appear in a specific order — the torch will appear before the (very, very good) sword will appear. Later games made all Power-Ups random, although they include many more let downs, too.
Recurring Boss: The original game has significantly fewer distinct boss types than there are levels.
Also Astaroth comes back in every game, even when he's not the Big Bad anymore.
Satan: The recurring Big Bad of the series is Astaroth, and Satan is his lackey. In Ghouls 'n Ghosts, the Big Bad is Lucifer (Loki in the American Genesis version). In Super Ghouls 'n Ghosts, the final boss Sardius is called Samael in Japan. These are all separate entities. And in Ultimate Ghosts 'n Goblins, Astaroth is still there as the Final Boss, but there's a True Final Boss after him: Hades.
There's also a demon bee boss named Beelzebub, which is sometimes another name for Satan.
Save the Princess: Every game is about Arthur rescuing Princess Prin Prin. Even in the second game, where she's killed by Lucifer, it's Arthur's goal to rescue her soul and bring her back to life.
Sequel Difficulty Drop: Ghouls 'n Ghosts is considerably easier than Ghosts 'n Goblins (though still Nintendo Hard), thanks largely to Arthur gaining the ability to throw weapons vertically and (with the Gold Armor) use magic.
Single Use Shield: In Ghosts 'n Goblins and its sequels, the player character's armor disappears after one hit, but they do have the benefit of starting each life with armor equipped. Super Ghouls 'n Ghosts also offers a shield powerup which can block one projectile attack only.
Spiritual Successor: Maximo: Ghosts To Glory is technically not part of the series, but was very obviously inspired by it. MediEvil too.
Super Drowning Skills: The numerous small pools of water in Arthur's path might as well be filled with lava or Spikes Of Doom. Averted in Makaimura for Wonderswan, where Arthur actually can swim.
It makes sense when he has armor, but WHEN HE'S IN HIS BOXERS?!
Theme Naming: Some of the members of the Red Arremer family are themed after playing cards, with Red Arremer King in Ghouls 'n Ghosts, Red Arremer Ace in Super Ghouls 'n Ghosts, and Red Arremer Joker in Super Ghouls 'n Ghosts Advance. Now where's Queen and Jack?
The Unfought: Satan appears in Super Ghouls 'n Ghosts, where he abducts Prin-Prin in the game's opening sequence, but he never actually fights the player as an enemy.
Unwinnable: The NES version had some bosses immune to some attacks. If you had the wrong item, you can't even get the screen saying you didn't get the shield.
The forth boss in Ghouls 'n Ghosts is a stationary slug. It can become this if Arthur happens to be carrying the short-range broadsword.
Updated Re-release: Goku Makaimura Kai for Goku Makaimura (Ultimate Ghosts 'n Goblins), which includes the original game and a revised mode that drops the RPG elements for a more Arcade-like experience. (Sadly, it's Japan-only.)
The Gameboy Advance port of Super Ghouls n' Ghosts could also count as this. It includes an all-new "arranged mode" with branching paths and new levels.
Wallmaster: Some enemies appear uncomfortably close.
Weaponized Offspring: In Super Ghouls 'n Ghosts, the boss of the first stage is a giant bird which spits eggs out of its mouth which hatch into smaller, groundbound bird monsters which attack you.
Womb Level: The fourth level in Super Ghouls 'n Ghosts is called "The Ghoul's Stomach." It's appropriately spooky.
Your Princess Is in Another Castle: THIS ROOM IS AN ILLUSION AND IS A TRAP DEVISUT BY SATAN. GO AHEAD DAUNTLESSLY! MAKE RAPID PROGRES! . After defeating the Ghosts 'n Goblins Final Boss, Arthur is forced to replay the entire game, on a harder difficulty level —of all things— to get the genuine end.