King of Sparta is the successful adventure film that Tristan McClean starred in. The poster for the film features Tristan wearing a kilt and, much to Piper's embarrassment, is hanging everywhere. "It was," she says, "the most ridiculous poster of all time" (15.22).
King of Sparta is, then, an incredibly popular fiction about ancient Greece, and in this way, it mirrors The Lost Hero (and Riordan's Olympus books in general), which is also extremely popular, and also works with material derived from ancient Greece. Tristan's film is a kind of story-within-the-story. The novel is winking at its own success and, perhaps, at its own heroic silliness.
Along these lines, when the heroes are fighting the giant Enceladus, Leo wishes they were in a Tristan McClean movie, and that Piper's dad could "untie his bonds and knock out the giant with some cleverly hidden anti-giant gas" (41.50). Leo presents this as impossible, but it's more or less what happens—except it's Jason and Leo and Piper who defeat the giant in incredible and unlikely ways. They are in a Tristan McClean movie, though they're the heroes. King of Sparta is to their world what The Lost Hero is to ours: a heroic fiction that it's fun to pretend is true, even though you know it isn't.