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  Both sides would, of course, make the same calculations, most likely arrive at the same answer. Long before Vauban wrote down the ratios in his impeccably scientific book, the basic formulae were well known to everybody in the trade. Of course, there were always mavericks who didn’t care, like the Zealots who defended Masada against the Romans, and killed themselves rather than surrender. A defending commander serving a psychotic autocrat (no shortage of them throughout history) might choose to continue an untenable defence if he knew that a sensible, businesslike surrender would cost him his head. Naturally, the headman’s axe cut both ways. Many unwinnable sieges dragged on because the besieging general knew exactly what would happen to him if he gave up and went home.

  Treachery and sweet reason both having failed, the besieger had three options. He could withdraw; he could sit outside the walls and try and starve the enemy out; or he could mount an assault in hope of carrying the city by storm.

  Torsion artillery—stone and arrow throwing engines powered by twisted-rope springs—started with the Greeks and overlapped with gunpowder. The Roman name for the rope-powered single-armed engine was onager, the wild ass, because the way the throwing arm slammed into the wooden frame reminded them of a donkey’s kick, or because the squeaking sound it made as the rope was being wound sounded a bit like ‘hee-haw’. It took eight men to work it. At Masada, the Jewish garrison got the hang of watching the trajectory of the white balls against the sky and getting out of the way just before they pitched; the Romans painted the balls black and started killing Jews in great numbers. The double-armed engine, resembling a giant crossbow but also powered by twisted rope rather than just the bending of the arms, was called the ballista, from the Greek word for ‘to throw’. It could shoot arrows or throw stones. In the Islamic world it was called a ziyar, and was used well into the 15th century. The Europeans refined the ziyar to develop the espringal, wound by a long screw and a windlass.

  Torsion engines killed men on both sides and caused a certain amount of damage to masonry, but they weren’t city-smashers. Trained artillerymen could shoot them surprisingly accurately. Their main function was to keep heads down (or knock them off). To get inside the city, the attackers had three options; to climb over the walls, bash down a gate, or breach the walls by digging under them and making them collapse.

  Climbing the wall, using long scaling ladders, was the least effective option if the wall was adequately defended. A man climbing a ladder is sadly vulnerable. You can drop things on his head, push the ladder away from the wall with a forked stick, or kill him as soon as he reaches the upper rungs. Siege towers evened the odds a little; mobile wooden fortresses as tall as the tops of the walls, with platforms for archers who could lay down a barrage of arrows. But the towers themselves were dangerous places to be. They were usually armoured with thick planks, sometimes with water-soaked hides to protect them from fire; they worked well for the Assyrians in the 9th century BC, and the Greeks called them helepoleis, ‘city-takers’, which suggests they got the job done, for a while at least. More sophisticated versions were fitted with a drawbridge arrangement, allowing men to cross from the tower directly onto the wall. The Turks were still using siege towers in 1453, against Constantinople, and in 1565, at the siege of Malta. But they were cumbersome, needing to be wheeled into position by teams of men or oxen; a good hit from an onager would cause havoc inside, and if they caught fire they were death-traps. If the approach to the city was rough, uneven or wet, getting a tower up to the wall must have been a nightmare. Defenders sometimes dug deep trenches around the walls, filled them with storage jars and covered them with turf; when the towers were brought up, their weight crushed the jars and the towers collapsed into the trench. The Greek architect Diognetus foiled the towers of Demetrius Poliorcetes (his name means “sacker of cities”) by commandeering the contents of every midden and chamber-pot in the city and pumping it out through holes in the wall; the towers duly bogged down and got stuck.

  Breaking down a gate was a basic approach. Battering-rams are unsophisticated things; a tree-trunk, with or without handles, a cradle of ropes to swing from, a covered frame to protect the men propelling the ram from arrows and rocks. The defenders would, of course, brace the gate from the inside, and in the medieval period, gates were often made of an early form of plywood—sheets of wood laminated together with the grain of the alternating layers running in different directions to stop them splitting. Rather more sophisticated than basic rams were worms, giant drills under covered frames for boring into the walls themselves.

  Against a sparsely or carelessly defended city, assaulting the walls or the gate might get the job done. A strong, intelligent defence stood a reasonable chance of success. The attacking force needed numerical superiority of at least five to one, and even then losses would be high. Attacking two or more points on the wall simultaneously made things harder for the defence, assuming the attacker had the numbers and the resources. Morale was the biggest problem. A man needed to be highly motivated before he could be induced to climb a long ladder under constant fire, with every chance of falling to his death or being killed as soon as he set foot on the parapet. Leaders often took great risks to inspire their men. Alexander the Great frequently led such assaults from the front; he was wounded several times, and got the wound that finally killed him leading an assault on a city in India.

  A much more businesslike approach, though slower and requiring vast effort, was undermining the walls. The basic technique was simple, and in use from the earliest times up to the First World War. A team of specialists dug a tunnel under the wall, shoring it up with wooden props as they went. When they reckoned they were directly under the wall, they dug a chamber and filled it with brushwood, which they then soaked in oil and set alight. The fire burnt through the props, the chamber collapsed and the wall above subsided and collapsed, leaving a breach through which the attackers could enter the city. Gunpowder made little difference; instead of brushwood, sappers stuffed the chamber full of explosives.

  Foundations of solid rock made sapping extremely difficult; moats meant the sappers had to dig deep or risk being flooded out, and the deeper the sap passed below the wall, the bigger the chamber had to be if it was to cause enough subsidence to collapse the wall. The Byzantines reinforced the foundations of some of their fortresses with stone columns to obstruct sappers. Where such reinforcement was lacking, the defenders’ options were limited to counter-sapping; figuring out or guessing where the enemy sap was, digging a tunnel of their own, breaking through and killing the enemy sappers or undermining their tunnel and caving it in. To locate the enemy workings, defenders placed bowls of water on the ground and watched for ripples caused by the vibrations, or hung bells on portable frames. In 265 AD, at the siege of Dura Europos in Syria, the besieging Persians dug a sap under the walls of the Roman fortress. The Romans countersapped, killed the Persians and collapsed the tunnel, creating a unique time capsule. Archaeologists found evidence of savage hand-to-hand fighting; discarded weapons, armour, bones, even partly-preserved bodies, along with the resin and sulphur the attackers had brought to ignite their chamber with. Hand-to-hand fighting in dark, cramped galleries and shafts was, naturally, something to be avoided if at all possible. Whenever they had the opportunity, the defenders relied on other expedients to drive the enemy sappers out of their tunnels. If they had running water, they had the option of flooding the sap; failing which, there was always smoke—Aeneas Tacticus, Alexander’s armchair tactician who also recommended bees, as noted above, was keen on straw smoke, but the most effective fuel was chicken feathers, which saved Ambracia from the Romans in 189 BC.

  Siege warfare from the earliest times to the beginning of the fourteenth century was, therefore, largely decided by resources. If you had the time, the men and the equipment, you could be fairly sure of taking a city by undermining the wall. The cost in time, lives and money was enormous, and unless you were lucky enough to carry the city by a surface assault, you we
re faced with a cold, straightforward calculation. Could you afford the outlay, and if so, was the city worth it? If the answer was no, you had two choices; give up and go away, or blockade the city and starve the besiegers out.

  Blockade—a siege as opposed to an assault—may seem a rather more sensible, businesslike approach. Why go to all that trouble and effort when starvation will do the job for you? The problem was that a siege could easily turn out worse for the side outside the walls. The defenders, after all, had proper roofs over their heads, while the attackers were under canvas. Any governor, castellan or garrison commander worth his salt would have substantial reserves of food in store before the siege began. Supplying the besieging army was likely to be its commander’s biggest problem. Unless food could be brought in directly by sea, supply from home was fraught with difficulties. The besieging general had to contend with extended lines of supply, miserable and often impassable roads, ox-drawn carts moving at three miles an hour. More usually, he’d have no option but to try and live off the land, assuming that the enemy hadn’t taken the simple precaution of burning every ear of wheat and driving off every head of cattle. Even if his opponent had neglected these basic precautions, there were few places in a subsistence-agriculture world where there was enough surplus food lying in barns or stores to feed tens of thousands of outsiders for months, or years, at a time. A special delight reserved for the garrison of a well-supplied city whose besiegers were starving was to load their onagers with loaves instead of rocks and shoot them out over the enemy camp in a gesture of diabolical charity.

  The greatest danger to any army, regardless of which side of the wall it was on, was disease. Before the twentieth century, disease killed many times more soldiers than weapons ever did. The biggest killer was dysentery, also known as camp fever. Most cities had at least some rudimentary drainage and sanitation. A siege camp was a much more basic affair. If the siege took place in summer, the heat made things worse; winter sieges meant bitter cold, endless scrabbling for firewood, wretched cold under canvas. Rain at any time of year meant drenched clothes and the constant misery of mud.

  Kings and generals in their pavilions would not, of course, feel the pain quite so keenly, and since they made the decisions, the suffering of the besieging army wasn’t usually a good enough reason in itself to give up and go home. But when men started dying in unacceptably large numbers, the besieging commander might well contemplate renewing the assault, or stepping up his attempts to find a traitor inside the city. At the very least, if he had to call the siege off and go away, it would be on the understanding that he’d be back again as soon as the weather improved, or he’d refilled his ranks, or amassed the necessary supplies.

  In the early years of the fourteenth century, the first real improvement in artillery since the Romans was developed, probably by Muslim engineers working for the Mongol khans. The trebuchet was a stone-thrower, like the torsion engines. But it threw a bigger stone, and much further. Trebuchet shot could batter down walls, from a safe distance. Basically a giant see-saw, the trebuchet was an asymmetrical lever mounted on two A-shaped supports. The lower, shorter end of the lever connected to a counterweight, usually a huge box filled with rocks. The top end was the throwing arm, a long, slim shaft topped off with a sling. The crew hauled the throwing-arm down with winches to load the sling and raise the counterweight. A simple loop-and-hook release let it go; the counterweight dropped and the throwing end swung up like a baseball pitcher’s arm, discharging the payload with a final flick from the sling. It was simple, relatively easy to construct, and not constrained by the physical properties of rope. The only limit on the size and power of trebuchets was the supply of quality lumber and the skill of your carpenters.

  Defenders tried to compensate by building thicker walls and shooting back with trebuchets of their own. But the balance had shifted dramatically; at last, the attacker had technology that made a swift, cost-effective assault a realistic proposition. Before the fourteenth century was out, the trebuchet was itself on the way to being obsolete, as the first cannon started punching holes in masonry. The earliest guns looked like bells and shot arrows, and were cast in bronze; their development was hindered by the extreme difficulty of casting something as big as a cannon. As soon as molten metal flows into a mould, it begins to cool. Using technology fundamentally unchanged since the Bronze Age, early gunfounders discovered that the outside of a gun had cooled down while the inside was still hot. The result was fissures, flaws and cracks in the metal, which tended to blow apart with extreme prejudice when charged with gunpowder and let off.

  So gunmaking passed from the bell-founders to the blacksmiths, who bent sheets of iron round a mandrel and forge-welded a seam to make a tube, which they reinforced with stout iron bands, as coopers did with casks (hence the term gun-barrel). These guns, made in two parts with a detachable breech, were considerably more reliable. There was no new technology involved in their manufacture; rather, it was old technology employed on an unprecedented scale. I explained how such guns were made to one of the best blacksmiths in Europe and he didn’t believe me; not possible, he said, you couldn’t get that much metal up to an even welding heat, and even if you could, you wouldn’t have enough time to form sound welds. As for heating up the bands and then shrinking them onto the barrel, forget it. Too much heat, not enough time. Couldn’t be done. But it was, and the welded-and-hooped cannon made rubble out of walls that had defied assault for centuries. Later, gunfounders fixed the cracking problem by casting the cannon solid and boring it out with a huge lathe. Finally, an American inventor around the time of the Civil War figured out how to homogenize the temperature of the liquid metal and cast a gun which didn’t need drilling out of solid.

  Artillery changed everything. Instead of thick stone walls, cities were defended by broad earth banks, into which cannonballs sank harmlessly, and the defences were reshaped to allow wall-mounted cannon a maximum field of fire. A cannon mounted on a straight wall can’t shoot downhill; if you want to harrass the enemy once they’re close to the city, you need triangular projections (bastions) sticking out, from which you can enfilade. Cities started to look like starfish, and (thanks to canister—hundreds of musket bullets in a leather bag, turning the cannon into a giant shotgun) gunshot range from the battlements was a no-go area where nothing could survive.

  From the fifteenth century to the mid-nineteenth, therefore, sieges once again belonged to the sappers. The problem they faced was getting to the wall without being killed by artillery fire. The only way to do this was by digging very long trenches, which couldn’t be straight, because that would invite the enemy to lob balls and shells (another Mongol invention) into the trench, with appalling results. So the trenches zig-zagged their way across hundreds of yards, from out of shot right up to the wall. The sides of the trench had to be protected by gabions (large wicker baskets packed tight with earth, like sandbags), which absorbed a certain amount of shot and were then replaced.

  Defenders could launch sorties and kill sappers, block up trenches, collapse galleries with explosives, but if the enemy had the time and the resources, they were only delaying the inevitable. Once one of Vauban’s zig-zags appeared in front of the city, there wasn’t really anything anyone could do. As before, the only real constraint was the besieger’s ability to supply his army. Vauban’s employer, Louis XIV of France, was the richest king in Europe, and he bled France white paying for sieges. Being besieged wasn’t quite so ruinously expensive, but it didn’t come cheap. Of course, during the eighteenth and early nineteenth centuries, Europe was awash with new money—gold from the New World, the proceeds of expanded trade and early colonialism; just as well, given that Europe’s rulers had acquired such a taste for probably the most expensive hobby in history, at least before the start of the space race.

  It was in the New World, during the American Civil War, that siegecraft finally overextended itself and fell into decline. It was a war decided by a different sort of engineer; cutters
of metal, not shapers of stone. For example; at Petersburg, the Union forces executed a textbook undermining of the Confederate fortifications. Vauban would have approved. But, when the camouflet went up, a gaping breach appeared in the line and the infantry poured in to secure it—a formality, according to the rules—they were shot to pieces by the Confederates. Samuel Colt’s Hartford production line had revolutionised small arms manufacture, with the result that every infantryman now held a rifle capable of accurate aimed fire. At New Orleans, the Union used the waterways to float city-smashing mortars close enough to the city to prompt surrender. Paddle steamers and railways made possible a new kind of war, fought in the open over great distances, with mobility the key. Why bother cracking open the layered defences of Atlanta when you could simply go round them?

  The machine-gun briefly gave the battlefield back to the trench-diggers in the First World War, just as cannon had done in Vauban’s time. But the tank—a siege tower, if you like, but self-propelled and massively armed and armoured—put an end to trench warfare. The tank was, of course, nothing new. It was the logical development of the 16th century armoured wagons of Jan Huss. Leonardo had tried to invent it. With the advent of fast, powerful tanks, the siege as generations had known it was obsolete. Motorised warfare dealt with the Maginot line by going round it. A different kind of siege took place at Stalingrad. The pounding of masonry into rubble by long range artillery simply gave the defending infantry endless cover, and Vauban’s formality became the main event. Aerial bombardment brought the horrors of war back to city-dwellers in yet another permutation of the siege, but the Second World War was won in the open field, by mobility, firepower and (most of all) industrial capacity.