İsa Kapı Mosque was a Byzantine church in Constantinople dating to the Palaiologan period. It was located near the Column of Arcadius and the Mese, the main avenue of Constantinople. Its Turkish name, which means the “Gate of Jesus”, seems to mark the approximate location of the Constantinian Golden Gate – thus indicates the location of the Constantinian Land Walls. Its identity is unknown, though the Monastery of Patriarch Athanasius I and the Monastery of Iasites have been suggested. 
Only the eastern wall, parts of the apse, a small part of the southern wall, and the eastern corner of the north wall from the original building have survived. Its masonry is similar to Pammakaristos and Chora monasteries, suggesting it was constructed in the late thirteenth or early fourteenth century. It was a single-aisle, wooden-roofed basilica, measuring 20 x 8 cm, with a wide and long projecting apse, which was flanked by a prothesis and diakonikon.
By 1560, Hadım Ibrahim Pasha converted the church into a mosque. Mimar Sinan oversaw its conversion, building a madrasa around it. He replaced the apse to a flat wall, built a mihrab along the southern wall, and walled off the prothesis. Most of the building collapsed in the earthquake of 1894. Traces of frescoes, noticed by earlier scholars, have now all but disappeared. It was in ruins throughout the 20th century, though it was recently restored.

From Byzantine Topographic Studies by Paspates (1877)

From the Nicholas V. Artamonoff Collection

References

Architecture and Ritual in the Churches of Constantinople: Ninth to Fifteenth Centuries by V. Marinis
Converted Byzantine Churches in Istanbul: Their Transformation Into Mosques and Masjids by S. Kirimtayif
Byzantine Constantinople: Monuments, Topography and Everyday Life edited by Nevra Necipoğlu
Bildlexikon zur Topographie Istanbuls: Byzantion, Konstantinupolis, Istanbul by Wolfgang Müller-Wiener

Resources

Byzantine Churches of Constantinople Photo Album (Byzantine Legacy Flickr)

Byzantine Churches of Constantinople (Byzantine Legacy Google Map)

Isa Kapı Mescidi (Nicholas V. Artamonoff Collection)