The "Book of All Kingdoms", of 1350, tells the voyages of an anonymous Castilian friar and is illustrated with 113 flag images, referred to (though seldom described) in the text.
The 95th flag mentioned and illustrated in the "Book of All Kingdoms" is attributed to the "Empire of Constantinople", the Byzantine Empire).
The 2005 Spanish illustrated transcription of the "Book" shows a quartered flag, I and IV, white with a red cross, II and III, red with a yellow cross couped, with each of its quarters taken by a yellow chain link; the flag is shown in the ogival default shape of this source. The design of the II and III quarters is identical to four other Byzantine flags in this source.
The anonymous author of the "Book" describes the flag thusly: E el emperador de Costantinopla há por señales un pendón a cuarterones, los dos cuartos blancos con cruzes bermejas, e los otros dos cuarterones son bermejos con sendas cruzes de oro e con cuatro eslabones de oro d'esta manera (The Emperor of Constantinople has for device a flag quarterly, 1st and 4th argent a cross gules, 2nd and 3rd gules a cross or between four chain links or).
The depiction of the flag in the 2005 edition matches the one given in the Hakluyt Society edition (1912) of the "Book", which sources the same design to manuscript "S", while from manuscript "N" is shown instead a samnitic shield, thickly edged in black, quartered red and white with golden crosses fleury (?) on quarters I and IV and red such crosses on quarters II and III, the golden crosses added with two (not four) annulets (not links) each: on quarter I, on the II and IV cantons made by the cross, and on quarter IV, on the II and III cantons made by the cross.
Via CRW Flags