I think this illustration is in reference to the following:
The first target of the rebels was the former capital of the Trinovantes, Camulodunum (Colchester), which had been made into a colonia for Roman military veterans. These veterans had been accused of mistreating the locals. A huge temple to the former emperor Claudius had also been erected in the city at great expense to the local population, causing much resentment.[13] The future governor Quintus Petillius Cerialis, then commanding the Legio IX Hispana, attempted to relieve the city, but suffered an overwhelming defeat. The infantry with him were all killed and only the commander and some of his cavalry escaped.[14] The location of this battle is unknown.[15]
The Roman inhabitants sought reinforcements from Catus Decianus, but he sent only two hundred auxiliary troops. Boudica's army attacked the poorly defended city and destroyed it, besieging the last defenders in the temple for two days before it fell. Archaeologists have shown that the city was methodically demolished.[16]