Ships dragged over a mountain 

The problem was stated cleanly. The chain could not be forced. The Horn wall could not be attacked from water the fleet could not reach. So in the third week of the siege the Ottoman camp began building something odd on the slope behind the Genoese enclave at Galata: a graded track of timber, running from the Bosphorus shore up over the ridge and down to the inner waters of the Golden Horn.

The technique itself was not new. Slipways, greased ways and portages were standard in a Mediterranean world where galleys were regularly hauled out of the water for the winter, and there are earlier instances of fleets being dragged short distances overland. What was new was the scale and the speed. The sources describe on the order of seventy vessels moved, mostly light galleys and smaller craft rather than the heaviest ships, on wooden rollers or cradles lubricated with tallow and fat, pulled by oxen and by very large numbers of men.

Read practically, this is a labour problem rather than a marvel. Mehmed had tens of thousands of men in camp with time between bombardments, engineers used to building roads for the guns, and a treasury able to buy timber and grease. He also had the neutral zone of Galata to work behind, which shielded the operation from the city's view until it was well advanced. The Genoese of Galata were technically neutral and were trading with both sides, which is its own quiet story about the fall of a Christian capital.

The effect on the defence was immediate and disproportionate. Ottoman ships now floated inside the Horn, behind the intact chain. The defenders had to garrison the long, low harbour wall properly, drawing men away from the land walls where the breaches were opening. A plan to burn the transplanted fleet with fire ships was attempted and failed with heavy losses and executions on both sides afterwards. Worse, the Ottomans could now bridge and threaten the wall from floating platforms.

There is a temptation to call the portage the moment the city fell. It was not, quite. The cannon did the structural work and the final assault came at the land walls. But the portage is what turned a siege the defenders were surviving into a siege they could not staff, and it is the clearest single demonstration of what Mehmed brought to the campaign, which was not ferocity but a refusal to keep playing on ground the enemy had prepared.

Out at the walls, meanwhile, the bombards had settled into a rhythm the defenders were learning to live with.