As Muslims of the fledgling Islamic caliphate went on with their conquest of Egypt from the Byzantine Empire beginning in 639 AD, three years after their victory at the battle of Yarmouk in 636 AD, they conquered all of the land by the time they have captured the city of Alexandria in 641 AD. A year later in 642 AD (year 21 in Islamic calendar), they founded the city of Giza. Its name, "al-Jizzah" in Arabic, means "the valley" or "the plateau", pertaining to the area's topography. Peust also suggests a Persian etymology of the word from Middle Persian "diz" (fortress, castle), which Persians could give to the pyramids or a fortress they've found in the area.