After the rape of her daughters, her own lashing and the outright theft of Iceni lands at their Roman masters, Boudicca inspired an army of some 100,000 to break out from their oppressive yolk. Perhaps a more important factor, however, than any leadership qualities of the Iceni Queen, or feelings of vengeance among the Iceni, was the simple fact that the Legions were nowhere near the Iceni lands at the time of the uprising. Though word had reached the Roman governor Suetonius while on campaign at the Island of Mona (Anglesey), his march would take considerable time to counteract Iceni plans. Without local resistance of any note, Boudicca led her formidable army towards a colony of retired Roman officers at Camulodunum (modern Colchester).
Though the presence of settled veterans generally offered great benefit in the way of Romanizing an area, their presence here had the opposite effect. Inspired by vengeance against the soldiers who had wronged them, the Iceni stormed the practically undefended town. Though the Romans managed to hold out for several days, there was little hope for resistance or relief. The Procurator of Londinium dispatched 200 men to come to their aid, but this disproportionate reinforcement obviously had little effect. In the end, the town was razed and its inhabitants slaughtered allowing Boudicca to continue marching southwest to Londinium itself. As the town was virtually undefended, the Procurator, Decianus, fled with his staff, virtually leaving the Roman province of Britannia without a capital.
At this point, a reduced strength Legio IX Hispana had marched south from Lindum (Lincoln) under Petilius Cerialis but was obviously too late to help at Camulodunum. Likely pushing hard to cut off Boudicca before she reached the Roman administrative capital at Londinium, Cerialis walked into an ambush. IX Hispana, completely overwhelmed and outmanned, was nearly shredded entirely. The infantry was destroyed (likely around 2,000 men), but Cerialis managed to escape with the cavalry. The legion would later be reinforced by men from the Rhine, but for now, one complete legion was out of service, and there was little resistance in the path of Boudicca's march. However, it's possible that the slaughter of the Ninth may have allowed just enough time for Governor Suetonius to gather his forces and offer a unified defense. He arrived at the city before Boudicca, albeit with a drastically smaller force. With about 10,000 men, made up of detachments from Legio XX (later Valeria Victrix), Legio XIV Gemina (later Martia Victrix) and any auxilia he could gather, he approached and considered making a stand at Londinium. However, the city was a poorly fortified center of business and trade, and was ill-suited for making any such stand. Suetonius decided to abandon it taking with him anyone who could fight, while others certainly fled at his departure, leaving many more behind to meet their fate at the hands of the warrior Queen.
When Boudicca arrived, Londinium suffered largely the same result as Camolodunum, and was razed to the ground. The people were slaughtered and subject to all manner of reciprocal atrocity. The fire that took the city was so hot, that the melted remains formed a recognizable layer of red clay 10 inches thick in places, just below the surface of modern roads. Boudicca, still with her thirst for vengeance unquenched, left the burning wreck of Londinium behind and followed Suetonius towards the town of Verulamium (St. Albans). Again, he saw little opportunity to make an adequate defense and left the town to the enemy (perhaps hoping to buy time for more reinforcements, or to let the barbarians exhaust themselves on plunder) This time however, the inhabitants were well aware of her reputation and fled en masse. Still, Boudicca burned it to the ground just the same, and Tacitus estimates that some 70,000 people had been slaughtered in all between the 3 towns.. Though this is certainly exaggerated, Boudicca had already proved devastating to the fledgling province's ability to administer itself and thrive.
Meanwhile Suetonius, described by Tacitus as an officer of distinguished merit, attempted to give his small army a fighting chance. First he called upon Legio II Augusta (stationed at Isca Dumnoniorum, near modern Exeter) to join him in the forested Midlands near Verulamium, but its commander Poenius Postumus failed to show for unknown reasons (he later committed suicide as a result of the shame). Left with just his 10,000 men vs. what Cassius Dio described as a swelling army (unlikely) of some 200,000 under Boudicca; Suetonius positioned his meager force on high ground, with forested protection at his rear and flanks. The final battle vs. the Iceni Queen was about to begin.