"The Circular Ruins" is another one of Borges's stories involving a seemingly impossible, paradoxical interaction between an Object from the primary realm (here reality) and another one from the Secondary realm (in this story, a dream). It captures the themes from the three prior short stories in Ficciones - the one involving Tlon, the one involving Al-Mutasim, and the one involving Pierre Menard, each of which discuss the impact of fictional objects having on some aspect of reality (at least the reality within the context of Borges's story) forcing us to ask the question if the object that we consider to be in the primary realm isn't a representation of some other object in another realm!
Fascinating stuff.