Great cities, no less than small books, have their fates. Some are capable of self renewal from seemingly unsalvageable shards of their older phases, like Alexandria; some continue to add new rings to their girth, preserving alike the evidences of alternating greatness and mediocrity, like Cairo; some disappear entirely into memory and literature, like the Round City of al-Mansûr, which today has as little to do with its living descendant, Baghdâd, as do the City of Brass or Qur'ânic Iram. Despite an absolute dearth of archaeological spade-work on the site, and a correspondingly absolute dependence on written sources, the scholarly literature dealing wholly or in part with al-Mansûr's Baghdâd is by now fairly extensive. For the most part, this literature has restricted itself to discussion of technical and architectural problems, or to those of historical and social geography. This paper proposes to look into the symbology of the city's immediately striking plan, the cross within the circle, as another instance of the imago mundi, a fitting pattern for this capital of the world-bestriding 'Abbâsid Empire. In the course of investigation, some treatment, however summary, of a number of folkloric/literary motives was found requisite, all, it is to be hoped, tending toward the definition of a clear and plausible design.