Alright, breaking out my copy of Prokopios for this one.
Siege warfare in late antiquity, particularly by Barbarians, is detailed by multiple authors. The big ones are Ammianus Marcellinus, Priscus, and Prokopios, although there are others. But there is a fundamental problem with discussing siege warfare in Roman sources, and that is the Romans loved Thucidydes. And fundamentally, most Roman descriptions of sieges like to copy Thucidydes, which makes interpreting anything useful from them problematic. This is a big issue for both Priscus and Prokopios.
Our major accounts for barbarian siege activity (at least that involving engines and artillery) in the late Roman period revolve mostly around three major conflicts:
After the infamous Battle of Adrianople in 378 the Goths of Athanaric and Fritigern led an assault on the city of Adrianople which lasted for two days before they gave up the attempt. Ammianus Marcellinus describes the events in detail, stating:
And in order that no delays meanwhile might cool their ardour, at the fourth hour of the day they had encircled the walls and were engaged in a most bitter struggle; for the besiegers with their natural ferocity rushed upon swift death, while on the other hand the defenders were encouraged to vigorous resistance with might and main. And because a great number of soldiers and batmen had been prevented from entering the city with their beasts, they took their place close to the shelter of the walls and in the adjoining buildings, and made a brave fight considering their low position; and the mad rage of their assailants had lasted until the ninth hour of the day, when on a sudden three hundred of our infantry, of those who stood near the very breastworks, formed a wedge and went over to the barbarians. They were eagerly seized by the Goths, and (it is not known why) were immediately butchered; and from that time on, it was noticed that not a man thought of any similar action, even when the outlook was most desperate. Now, while this accumulation of misfortunes was raging, suddenly with peals of thunder rain poured from the black clouds and scattered the hordes roaring around the city; but they returned to the circular rampart formed by their wagons, and carried their measureless arrogance so far as to send an envoy with a threatening letter, ordering our men to surrender the city on receiving a pledge that their lives would be spared. 6 The messenger did not dare to enter the city, and the letter was delivered by a certain Christian and read: but it was scorned, as was fitting, and the rest of the day and the whole night were spent in preparing defensive works. For the gates were blocked from within with huge rocks, the unsafe parts of the walls were strengthened, artillery was placed in suitable places for hurling missiles or rocks in all directions, and a supply of water that was sufficient was stored nearby; for on the day before some of those who fought were tormented with thirst almost to the point of death.
Ammianus makes it rather clear that the Romans were not prepared for the initial assault. The walls had not been repaired in preparation for the siege, artillery was not constructed and set up, and there was no store of water to be rushed to the defenders throughout the day.
It's also rather clear that the Goths seem to have had little plan to assault the city initially. Ammianus makes no mention of ladders here, but they may have had them. It seems for the most part their assault was actually on the remnants of the Roman army trapped outside the walls, or possibly they were exploiting sections that were in disrepair to try and scale them there. Despite their successes against the Romans outside the walls, they were unable to breach them due to the hail of missiles and lack of equipment. As a result, they changed strategies:
The Goths on the other hand, bearing in mind the dangerous chances of war, and worried from seeing their bravest men stretched dead or wounded, while their strength was being worn away bit by bit, formed a clever plan, which Justice herself revealed. For they enticed some of our subalterns, who had deserted to them the day before, to simulate flight, as if returning to their own side, and to manage to be admitted within the walls, and when let in, secretly to set fire to some part of the city; in order that as if a kind of secret signal had been raised, while the attention of the throng of the besieged was distracted with extinguishing the flames, the city, left undefended, might be broken into. The subalterns when on their way as had been arranged, and when they had come near the moat, with outstretched hands and prayers they begged to be admitted, as being Romans. And they were let in, as there was no suspicion to prevent it; but on being questioned as to the plans of the enemy they varied in their answers. The result was that after being tortured in a bloody investigation they openly confessed with what purpose they had come, and were beheaded.
Trickery was the most effective way to capture a fortified city. For example, the Romans pretty much bribed or tricked their way into Constantinople in every civil war for the next thousand years, because it was too much effort to try and take it by force. However, the Romans had codified training for identifying deception and spies to counter this. As a result, the next day of fighting resorted back to a direct result.
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