A 21-year-old sultan and a Hungarian gunsmith 

Mehmed II became sultan twice. The first time he was twelve, installed when his father Murad II stepped back, and the arrangement lasted two unsteady years before the senior statesmen recalled the old sultan. The second time was 1451, when Murad died and Mehmed returned to the throne at nineteen with something to prove to a court that had already removed him once. Taking Constantinople was the proof available.

Two Ottoman attempts on the city, in 1394 and 1422, had failed against the land walls. Mehmed's approach was to change the terms rather than repeat the effort. He built a fortress, Rumeli Hisari, on the European bank of the Bosphorus opposite an existing Ottoman castle, so that the two together could close the strait and cut the city off from the Black Sea grain and Genoese shipping it depended on. That was finished in 1452. Then he went looking for artillery.

The engineer who supplied it was Orban, a founder from Hungary or thereabouts, and the most quietly astonishing fact of the whole siege is that he approached the Byzantines first. The sources indicate the emperor's government could not meet his price, gave him a small allowance and then failed to pay even that. Orban went to Edirne, where Mehmed asked whether he could cast a gun capable of breaking the walls of Constantinople. He said he could, and was given money, metal and labour on a scale the empire he had just left could not have imagined.

The result was a bombard reported at over eight metres in length, throwing stone balls said to weigh in the region of half a tonne. Contemporary figures for such guns are enthusiastic and should be treated as approximations, but the physical evidence of surviving Ottoman bombards makes the general scale credible. Moving it to the walls took teams of oxen, hundreds of men and weeks of road preparation. It could be fired only a handful of times a day, because the barrel had to be cooled and rested to keep it from cracking. It did crack, and was repaired in the field.

None of this was Ottoman secret knowledge. Gunpowder artillery was a European craft, developed in the same foundries that cast church bells, and available to whoever could pay the founders who knew how. That is the real lesson in Orban's career. Technology does not stay loyal to a civilisation. It follows the payroll, and in 1452 the payroll was in Edirne.

The defenders still had one weapon of their own that no cannon could touch, floating across the mouth of their harbour.