The Scarlet Citadel was the second Conan story to be published in Weird Tales magazine and involves an older, wiser Conan as king of Aquilonia. King Conan receives a call for help from Amalrus, the ruler of neighbouring Ophir, who claims that Strabonus, the Emperor of Koth, is threatening his kingdom.

When Conan marches into Ophir, with an army of five thousand Aquilonian knights, his planned campaign is revealed to be a trap as the two monarchs are working together to destroy him with the assistance of a Kothian wizard named Tsotha-lanti. The Aquilonian knights are driven-off by a swarm of giant crabs while Conan, having been captured, is imprisoned within a Korshemish dungeon. This dungeon is used by Tsotha-lanti for nefarious experiments, and Conan discovers many of the bizarre horrors during his escape.

Eventually, Conan frees Pelias, a former rival wizard of Tsotha-lanti, who helps him escape the dungeon and regain his position as king of Aquilonia. This notably makes Pelias a rare case of a sorcerer who is Conan's ally rather than his enemy (the only other being the witch Zelata in The Hour of the Dragon). The story climaxes with a gigantic battle, were Tsotha-lanti meets a grisly fate at the hands of Pelias.