The citadel of Cairo is one of the largest and most important citadels in Islamic military architecture. In spite of the numerous studies written about this citadel, the issue of the exact boundaries of the southern section of the citadel during the Ayyubid period has not been satisfactorily resolved. This paper will propose a reconstruction of the boundaries of the southern section of the citadel of Cairo at the time of its initial construction by Salah al-Din al-Ayyubi. Several scholars like Casanova, Creswell, Rabbat and others have suggested plausible reconstructions based on excellent analysis of both archeological and textual evidence. Based on textual and archeological evidence, this paper will argue that the southern walls of the citadel covered roughly the same outline as we know it today, covering the full boundaries of the rock on which the citadel was constructed. This is in direct contradiction to the arguments given by the scholars mentioned above, since all their discussions negated this possibility. The scholars' reconstructions were based partly on the analysis of the numbers given in a description by Salah al-Din al-Ayyubi's secretary, `Imad al-Din al-Isfahani, regarding the length of the walls constructed to encircle Cairo and Fustat including the Citadel in between. As part of the argument, this paper will show that the wording of `Imad al-Din al-Isfahani's text was either incomplete or inaccurate. Careful analysis of that text will lead us to only one conclusion: that there was a "missing" segment of the walls of the citadel of unknown length, making its perimeter larger that what the scholars have suggested. As a second part of the argument, this paper will document overlooked segments of walls and retaining walls that exist to date. We will provide enough proof to confirm the Ayyubid dating of those segments, strongly suggesting that those walls were part of the original Ayyubid boundaries of the citadel, thus forming the "missing" boundary mentioned above.