In Ghosts 'n Goblins, Arthur's armor is easily breakable, it's armor, why would it break like that? It's because he's fighting demons, of course they'll be able to break the armor!
It's actually been shown that when Arthur fights non-demons, his armor still shatters in one hit. However, the flipside to this means that the armor will take ANY HIT before breaking. Shown to full effect in Project X Zone.
Why is Arthur half-naked with Prin-Prin in the middle of a graveyard?!
Though it could be that the armor isn't very comfortable?
Why is Arthur with Prin-Prin in the middle of a graveyard?!
The second level of Ghouls n' Ghosts has Arthur coming into a village that's being burned to the ground. He then comes across giant piles of human skulls before entering. That the demons use as projectiles against him.
Arthur owns a horse in Ultimate G'nG, yet he chooses to walk the entire way to Astaroth's lair?
Considering in the arcade version of Ghouls n' Ghosts that nearly the first thing to happen upon returning after three years to save the kingdom is his horse getting disintegrated right before Prin-Prin gets shot as well, he probably didn't want to risk it.