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This is within Nilüfer Hatun imaret, originally a dervish frat-house then a 14th century soup-kitchen for the poor. It is contemporary with the Green Mosque, so early Ottoman, but its overall external appearance has an unmissable Byzantine influence, perhaps paying tribute to the origins of its namesake: Nilüfer Hatun, the daughter of a Byzantine governor, and the wife of Orhan. It was restored in 1955 but is now closed for further restoration. You may be able to peek at the marble sarcographi, columns, and columnheads in its yard.
Ayasofya Cami
This is a redbrick Byzantine-style basilica of 1065 AD, built over earlier churches back to the 4th century. (Its predecessor hosted the Seventh Ecumenical Council in 767, as their first meeting place at the palace was tumbledown.) Its name and status have long acted as political litmus. It was converted to a mosque in 1331 in the early days of the Ottoman dynasty, and restored in the 16th century after a fire. It suffered another fire in the war of 1920, and in 1935 was designated a museum. Having soaked up restoration funds to that purpose for over 70 years, it was re-opened as a mosque in 2011.
This is modern as the 15th century original was destroyed in the war, but it's built around a minaret of perhaps 1518, with tile-work similar to the earlier and better Green Mosque.
Yeşil Camii
Built in the 1380s, this mosque has an impressive 25 m minaret covered with green-turquoise-blue mosaics and tiles. It was shot up in the war of 1920 but restored in the 1960s.