During the Siege of Alexandria, Julius Ceasar and a few thousand troops were stuck in the royal quarter of Alexandria for several months, with a local army surrounding them.
Did the royal quarter have a massive stockpile of grain (enough to feed thousands of soldiers for several months), or did they somehow get supplies past the besieging army (perhaps through the port?)
Edit: Thanks, @Evargalo for the quote on the reinforcements from his Thirty-seventh legion. How did that shipment make it through the siege? Was there only one shipment, or several? If one, was that enough to last for all the months?