Cairo
The second translation capital and the medieval Mediterranean's largest market.
Kernel
Founded by the Fatimid caliphate in 969, Cairo became the Mediterranean's largest city — perhaps half a million people by 1300 — and the hinge connecting the Indian Ocean spice trade to Venetian and Genoese merchants. Al-Azhar (970) is the oldest continuously operating university in the world. When Baghdad fell in 1258, Cairo absorbed much of the scholarly diaspora.
Why it rose
Nile delta agriculture; the only major delta-port positioned between Indian Ocean commerce and Mediterranean shipping; Fatimid then Mamluk military protection; and a tax system that monetized the Red Sea spice route. Cairo was structurally the choke-point that the geography of medieval trade dictated.
What it gave the world
Al-Azhar's century-by-century continuation of the translation tradition. Mamluk navigation manuals that quietly seeded Portuguese ocean navigation. Refinements of al-Khwarizmi's mathematics. The Mamluk-Venetian commercial axis that fed Renaissance Italy's appetite for eastern goods until Portuguese Cape navigation broke the monopoly in 1498.
Why it declined
Vasco da Gama's 1498 rounding of the Cape of Good Hope broke the spice-route choke-point. Portuguese and later Dutch ships could carry Indian pepper directly to Lisbon, bypassing Cairo entirely. The 1517 Ottoman conquest folded Egypt into a larger empire and moved its capital function to Istanbul. Cairo remained populous; it stopped being central.