While Mohammed awaited more cannon, he ordered his soldiers to begin the work of filling the moat. For days, dawn to dusk, the Turks labored at this task. Each man carrying a bundle of sticks, a barrel, or a length of tree trunk; groups of Turks would rush to the moat in a hall of shots, stones and arrows from the walls, dump their burdens and scamper back to their trenches. Small guns from the towers discharged volleys of five to ten lead shots the size of walnuts against these laborers. Side by side with these modern armaments the mechanical engines, ballistas and catapults were still employed to discharge boulders, darts and Greek fire. The task of the Turks was made more arduous by heavy spring rains that churned the trampled ground to a quagmire.

A few times the defenders sallied from the city to harry this work, and they claimed to kill 20 Turks for every man they lost. However, it was soon decided that even this ratio of kills was unprofitable when measured against the huge numbers of enemy. From then on they stayed behind their walls during the day, but at night they crept out and worked like ants clearing as much debris from them as they could manage.

During these first days the Sultan's navy made its first attack against the boom stretched across the Golden Horn entrance. The attack was easily repulsed. Baltoghlu decided to wait for the arrival of his Black Sea squadrons.

Restless with the waiting, Mohammed took some of his best troops and attacked two nearby Greek forts. The garrisons were soon overcome. The seventy-six survivors were herded before the walls of Constantinople. There they were impaled alive on wooden stakes. It was Mohammed's warning to those who opposed his might.

By April 12th all the cannon, fourteen batteries of some four guns each, had arrived and been positioned to the Sultan's satisfaction. Now the bombardment of Constantinople began in earnest, a relentless booming and pounding that never ceased.

Artillery was still primitive, and a cannon was an unpredictable monster. After each ear-shattering boom and belch of acrid black smoke, a gun would recoil violently on its platform of planks, often lurching into the mud. Immediately oil was poured down its barrel to cool the scorched metal lest it split. Then with much straining and grunting the cannon was realigned on its target. When the barrel cooled the muck was swabbed out. Then began the process of priming with powder - an exasperating task in a gusty downpour of rain and loading the shot.

But each laborious volley heaped rubble at the base of the walls. Christian engineers quickly convinced the Turkish gunners of the effectiveness of concentrating their fire at the opposite sides of a bastion, at the inner angles where the structure jutted out from the wall.

The greatest voice in the hellish cacophony was Urban's colossal cannon. It was flanked by two other guns of somewhat slighter dimensions. About, a year earlier the Hungarian engineer Urban had offered his services to Constantinople. But the emperor could not afford the wage he asked, much less finance the project. Mohammed had seized the opportunity when Urban turned to him, granted the engineer four times the wage he asked and provided every facility. Within three months Urban cast the bronze monster whose proportions for those times were awesome. The barrel was nearly twenty-seven feet long. The bore was about twenty-four inches in diameter. Primed with a 134 pounds of powder, it could hurl a stone ball of more than 600 pounds about a mile.

The behemoth had been manufactured at the Turkish capital of Adrianople and had been dragged for two months the 150 miles to the battlefield on a platform supported by 30 wagons drawn by 60 oxen assisted by 400 men. The preparation for a single shot were such a major engineering operation that the gun could not be fired in one day more than seven times. But each deafening blast from its barrel, which shook the ground for a radius of four miles, hurled massive destruction against Constantinople's walls. To lessen the impact the defenders hung sheets of bullhides and bales of wool over the walls, but this stratagem proved futile.

At first the cannon was used against the Blachernal, but a few days later it was moved to a position before the Mesoteichion. Early in the siege it misfired, and in the explosion its creator was killed.

After a week of continuous bombardment, the Outer Wall of the Mesoteichion lay in ruin in many places. Giustiniani, however, resourcefully contrived to jerrybuild stockades of wood and earth to fill the gaps. These ramshackle but adequate defenses were constructed at night. Even the women of the city joined in this desperate work.

Meanwhile Baltoghlu launched a second attack against the harbor boom this time supported by his Black Sea squadrons. He now commanded a force of six triremes, all two-masted with their rowing benches on one level so the ships sat low in the water, ten biremes, nearly a hundred galleys and swift fustae (long-boats with single banks of rowers), and a myriad of smaller craft. By no means were all the rowers dispirited slaves and convicts - many men volunteered to the rowing banks, attracted by the very high pay.

The main power of the fleet of Constantinople was twenty-six major fighting ships, nearly all high-decked and powered solely by sail. Constantine had assigned ten of these vessels to anchorages outside the great chain boom to guard the harbor entrance which was about a quarter mile in width.

With his largest vessels Baltoghlu approached the boom and the guard ships. The Turks fired a salvo of cannon balls, then a hail of arrows. Then when they were near enough the Turks hurled flaming brands at the Christian ships. When the two fleets joined the Turks charged forward with grappling irons and ladders.

But ships cannons in those days were not powerful enough to damage the stout planking of large vessels. Nor could the guns on the low-lying Turkish vessels gain enough elevation to rake the enemy's decks. The Christian crews easily doused the burning brands with well-organized bucket brigades. Meanwhile the Christian marines from the vantage of their higher decks inflicted heavy casualties on the Turks with arrows and javelins. At the same time their stone-throwing machines scored repeated hits on Baltoghlu's own vessel causing considerable damage. Slowly the Christian ships began to encircle the Turks. Baltoghlu finally saw the danger and ordered a retreat.

When these tidings were brought to Mohammed he was angry and humiliated. But he coolly assessed the situation and ordered his engineers to modify the Turkish naval guns to achieve a higher elevation. He also moved some of his land cannon to Galata Point. These bombarded the Christian ships patrolling the boom, sinking one. The remaining watchdogs were forced to withdraw to positions inside the boom where they were sheltered from the land guns by the walls of Pera. Even so, Mohammed was reluctant to risk his fleet again. He determined to take Constantinople by land.

Two hours after sunset on April 18th, the Turks suddenly attacked the rickety palisades of the Mesoteichion. Urged on by the tattoo of drums and the clanging of cymbals, bearing lighted torches and shouting their battle cries, Janissaries charged across the filled moat. They were supported by heavy infantry, javelin-thrower and archers. The flickering lights revealed a scene of desperation and confusion as the Turks set the wooden palisades afire; they strained with hooks fixed to their lances to pull the palisades down; elsewhere with ladders they scaled the walls and for the first time Turkish steel clashed with Christian in desperate duels. Giustiniani was always in the thick of the fighting, rallying the men when they faltered and inspiring them with renewed energy and courage. Wave after wave of Turks maintained the pressure of the assault for four hours before they admitted defeat.

Once again Mohammed had been humiliated. His losses were estimated at 200 men while not one Greek - all well armored - had been killed. The bombardment continued, but there was a new spirit of hope now in Constantinople. If only help would come from the West the city might Still be saved.