The Iceni were a tribe of Celtic origin that occupied an area roughly that of present day Norfolk & Suffolk at the beginning of the first millennium.
Julius Caesar was the first Roman to invade Britain in 56BC, but it wasn't until nearly a century later that Rome actually took control of England with the invasion by Emperor Claudius in 43AD. While some of the tribes came under direct rule of the Romans, other tribes (including the Iceni) came to terms with the invaders and became in effect vassal territories under Rome.
The King of the Iceni was Prasutagus who during his reign amassed considerable wealth.
The Roman procedure at the time was that if a vassal king died the Romans took over the area and the benefits of ownership. Prasutagus tried to side-step this by leaving instructions that on his death, his lands and wealth should be equally divided between his family (his wife Boudicca and his two daughters) and Rome.
Prasutagus died in 60AD. When the Romans heard of his will they moved in to take possession of the wealth and territory. Boudicca was flogged, her daughters raped, and the Iceni hierarchy treated like slaves.
One must assume that the Romans only left a small detachment in the area because before long Boudicca had rallied the Iceni, raised an army and was marching south towards Colchester, then the Roman capital.
At the time most of the Roman army was in North Wales where the druids were making a last stand in Anglesey. There was a small detachment at Peterborough, and when they heard of Boudicca's uprising they marched towards Colchester. However, Boudicca seems to have had a good scouting / intelligance system working because she sent part of her army to meet them, and the Romans were defeated long before they reached the capital.
At Colchester the Romans had forsaken building fortifications to concentrate on building a large temple dedicated to Emperor Claudius as the first major undertaking in their new capital city. With no defences in their way the Iceni stormed into the town. They took no prisoners. They killed everyone in their way and set fire to the town.
They then carried on to London, which was then a new settlement. Again the thousands who had rallied to Boudicca destroyed the town and killed its inhabitants. A detachment of Roman cavalry, sent ahead of the main army marching down from Anglesey, saw the destruction as they approached. Realising they were no match for the Iceni who ahd turned to face them, they retreated back along the Watling Road to join up with the main army. The Iceni gave chase, stopping long enough to destroy the town of St. Albans as they passed through.
The main Roman army was making its way back from Angelsey along the Watling Road. Suetonius knew he was heavily outnumbered, so he selected a place for the battle and waited for the Iceni to come to him. With dense woodland protecting his rear and a narrow defile in front, he was in a position where his troops (outnumbered by 10 to 1) had the advantage.
The Iceni, so confident of victory, charged down the defile falling over each other in the charge. The Romans stood firm. The Iceni charged again and again, but they were no match for the diciplined Romans, and in the end the Romans charged against the Iceni trapping them against the waggons that had followed the army. By the end of the day 80,000 Iceni lay dead.
Boudicca and her daughters escaped, and later poisoned themselves rather than face the humiliation they would have suffered if captured.
The Romans then built the town of Venta Icenorum (just south of Norwich) as the capital of the Iceni area.