Alexander the Great chose the site in 331 BCE, on a strip of land between the Mediterranean and Lake Mareotis, and left his architect Dinocrates to lay it out on a grid; Alexander himself never saw the finished city, and later tradition held that his own body was eventually entombed within it. Under the Ptolemies, the Greek dynasty that inherited Egypt after Alexander's death, Alexandria became the largest city in the Mediterranean world. Ptolemy I Soter promoted the cult of Serapis, a deliberately invented god combining the Egyptian Osiris-Apis with the iconography of Greek Zeus and Hades, to give Greek settlers and Egyptian subjects a shared object of worship. The Serapeum, his temple, grew into one of the largest religious buildings of antiquity, standing on high ground in the city's western quarter.
Beside it stood the Museion, "the shrine of the Muses," a religious foundation in its own right as well as a center of scholarship, and its associated Library, the most famous collection of texts in the ancient world. Both declined in stages, damaged in the fighting during Julius Caesar's siege of 48 BCE and further diminished under later Roman rule. The Serapeum itself came down deliberately: in 391 CE, following imperial edicts against pagan worship under Theodosius I, a Christian mob led by Bishop Theophilus destroyed it, ending Serapis's cult at its own center.