The Mosque of Yıldırım, the earliest of all the existing mosques in Edirne, is a building whose past goes beyond the Ottomans and which presents us with a number of uncertainties owing to alterations accrued at various times. According to Dr. Osman Rıfat, the mosque was erected on the ruins of the Church of Tiris lye Hares. Gurlitt, noting the resemblance between the mosque and the Mausoleum of Galla Placidia in Ravenna, contended that the original structure was built before the Crusades, probably at a date not too distant from that of the Mausoleum of Galla Placidia (440 A. D. ?), and that it was converted into a mosque around 1400. The plan of the mosque as drawn by Gurlitt does, in fact, exhibit a close relationship to the plan of the Mausoleum of Galla Placidia, if we disregard the scale of the two buildings and that the former is in the form of a Greek-cross and the latter of a Latin-cross. However, the plan and section drawings published by Gurlitt do not present an accurate picture of the mosque as it is today —or as it was when Gurlitt studied it— but rather project his opinion pertaining to the original condition of the mosque. Gurlitt himself states that the drawings may be incorrect and that he could not vouch for the accuracy of the barrel-vaulted entrance hall. Indeed, the eastern arm of the cross, which serves as the vestibule, is different from the other three arms, which means that the interior space of the Mosque of Yıldırım in Edirne is not in the form of a perfect Greek-cross but of an asymmetrical cross where one of the arms is narrower and lower than the others : an arrangement of particular interest from the standpoint of early Ottoman-Turkish architecture.