Short answers, pulled from the story.
A census taken in late March recorded only 4,773 Greek defenders and roughly 200 foreigners within the walls. The population had collapsed from millions to perhaps fewer than 50,000 souls living inside the ancient perimeter.
Mehmed II began construction of Rumeli Hisarı fortress in early 1452 on the European side of the Bosphorus. This new fortification sat directly across the strait from the Anadolu Hisarı fortress built by his great-grandfather Bayezid I.
Giovanni Giustiniani arrived in January 1453 with 400 men from Genoa and 300 from Genoese Chios. He received overall command of the land walls defense from Emperor Constantine XI.
On the 20th of April a flotilla of four Christian ships managed to enter the Golden Horn after heavy fighting despite the chain barrier. Mehmed stripped Admiral Baltoghlu of his wealth and ordered him whipped 100 times for failing to prevent this breach.
Migration waves of Byzantine scholars émigrés following the sacking of Constantinople are considered key revival of Greek Roman studies that led development of Renaissance humanism science. These émigrés brought far greater preserved accumulated knowledge Byzantine civilization to Western Europe according Encyclopædia Britannica many modern scholars agree exodus Greeks Italy marked end Middle Ages beginning Renaissance.