The history of the siege remains very much alive in the modern city of Istanbul. You can still walk the full length of the land walls and, at certain points, you can climb atop them and look out upon the plain where Mehmed's army once sat. Near the middle of the walls, keep your eyes open for the gate which is named after Ulu, who really was the first Turk to breach the city walls. You can explore the ruins of the Blachernae palace. Gennadius's monastery — the Church of Saint Saviour Pantocrator — still stands, although it is now the mosque Zeyrek Camii. The cisterns underneath the mosque have been filled in, but you can visit another Roman cistern near the modern bazaar which encompasses the 'Street of the Colonnade', where William was held prisoner upon his arrival in the city. And, of course, the Haghia Sofia still stands as the glory of Istanbul.
The Topkapi palace did not exist at the time of the siege, but is well worth a visit. When you enter through the Imperial Gate, look directly above you for the seal of Mehmed, who began construction of the palace shortly after conquering Constantinople. The inscription above the seal reads: 'By the Grace of God, and by His approval, the foundations of this auspicious castle were laid, and its parts were solidly joined together to strengthen peace and tranquility… May God make eternal his empire, and exalt his residence above the most lucid stars of the firmament.' The palace was greatly expanded by later sultans, but its basic layout remains the same as in Mehmed's time. In the palace's portrait gallery, there is a painting of Mehmed by the Venetian Gentile Bellini. You can also visit the palace harem, a more ornate version of the structures found in Edirne and Manisa during Mehmed's time.
While the framework for the story is fact, Siege is a work of fiction and should be read as such. The personalities, motivations, plots and loves of the characters are fictional. Longo's quest for revenge is my addition. While a princess Sofia did exist, the particulars of her character and certainly her affair with Longo are fictional. (What is true is that in 1469 a young Byzantine princess named Sofia — her daughter, in my story — married Ivan the Great, the first Tsar of Russia.) History tells us that Halil really was at odds with Mehmed and Gennadius was a firm opponent of the Union of the Catholic and Orthodox churches. However, the plots and conspiracies they hatch in Siege are of my own invention. The Turkish harem certainly was a place of intrigue, a strange mix of people where everyone strove to rise from jariye slave girl, to odalisque at the court of a favourite, to lover of the Sultan and perhaps even mother of one of his sons. Mehmed really did have his young rivals drowned when he came to power, and there are stories of unfortunate harem women being placed in sacks and thrown into the sea. Again, however, the particular plots of Sitt Hatun and Gulbehar are of my own imagining. In this spirit, all of these characters should be treated as fictional, as should their story.