Borges hearts labyrinths. In fact, a labyrinth or maze tends to show up in pretty much all of his most famous short stories. (Check out "The Garden of Forking Paths" for a prime example.)
Let's take a look at our two labyrinth references in "The Secret Miracle":
First, as Jaromir exits his prison cell on the way to his execution, instead of seeing "a maze of galleries, stairways, and wings," he encounters "a single iron staircase" leading to a courtyard (9). There's only one way to go, and that's down the stairs to his doom. In his mind, he'd imagined an infinite maze of possibilities, but in reality, there's only one way out. Sounds kind of like Jaromir's fate, right? He really only has one future: and unfortunately for him, it's death (no matter how long he tries to put it off).
Second, our narrator describes the work that Jaromir composes in his head as a labyrinth: "Painstakingly, motionlessly, secretly, he forged in time his grand invisible labyrinth" (12). The comparison of Jaromir's play to a labyrinth suggests that literature ain't no straightforward narrative. And boy is that the case for our main man Borges: his works always send our minds spiraling down multiple paths, thanks to his provocative ideas and references to other works. Not bad, Borges.