Been a while since I read it now (still taking a little break from the Howard, reading Lovecraft on my commute, so I don't get too far ahead in my reading compared to my posting about the Conan stories), so last night I paged through the story again just as a refresher.
The Scarlet Citadel is the second story Howard wrote to feature King Conan, but it shares quite a few features of the other adventurer Conan stories Howard had been writing after The Phoenix on the Sword. The story begins with Conan the sole survivor on his side of a massive battle (again). This time, the kings of Ophir and Koth, working under the direction of the wizard Tsotha-lanti, have lured Conan and his Poitanian cavalry into a trap. Rather than have archers or hordes of spearmen kill Conan, Tsotha-lanti personally comes up and uses his "magic" (a poisoned ring in this case) to paralyze Conan, and take him prisoner.
Conan of course refuses the ridiculous offer the kings make him to give up his throne, so he's tossed in the dungeons under Tsotha-lanti's fortress. He escapes, wanders the labyrinthine passages until he meets a rival wizard, Pelias, and then with the help of Pelias' magic, returns to Aquilonia to battle it out with the usurping nobleman in league with the two kings.
It's a pretty good story. Not top tier Conan, but still really good. It's also dripping with D&Disms. Tsotha-lanti seems to be the sort of wizard who has some actual arcane power, but really just uses science when he can (Clarke's Law and all that) since it's easier. Still, the poison ring trick seems like a Hold Person spell in action (or maybe Hold Monster, since Conan is Name Level). The dungeons under the eponymous Scarlet Citadel appear to be a fairly standard megadungeon full of monsters and oddities, built by ancient pre-human civilizations and discovered by Tsotha-lanti and used for his own purposes.
I can easily imagine Dave Arneson and his buddies, playing out some Hyborian wargames using Chainmail, and someone suggesting, "Wouldn't it be cool if we played out Conan's escape from the Scarlet Citadel with the Man-to-Man rules?" Pure speculation on my part, of course, but it was likely stories like this one that helped inspire the jump from wargames to role playing games.