Even then,I don't think the Siege of Constantinople fits the bill either.The Ottomans definitely didn't have 300k on their side.Most sources cites somewhere along 70,000-150,000.I'd say that one of the battles from the Roman Civil Wars probably fits the bill.The battles around the period of the Tetrarchy were absolutely massive.Each side sometimes have something like over 100k.
I did not claim they have. But even going with only something like 100,000 besiegers, that's probably the largest concentration of troops for a single army in the Middle Ages in Europe.