Dictionary of Greek and Roman Geography/Iceni
ICENI, in Britain. Tacitus is the only author who gives us the exact form Iceni. He mentions them twice.
First, they are defeated by the propraetor P. Ostorius, who, after fortifying the valleys of the Autona (Aufona) and Sabrina, reduces the Iceni, and then marches against the Cangi, a population sufficiently distant from Norfolk or Suffolk (the area of the Iceni) to be near the Irish Sea. (Ann. xii. 31, 32.) The difficulties that attend the geography of the campaign of Ostorius have been indicated in the article. It is not from this passage that we fix the Iceni.
The second notice gives us the account of the great rebellion under Boadicea, wife of Prasntagus. From this we infer that Camulodunum was not far from the Icenian area, and that the Trinobantes were a neighbouring population. Perhaps we are justified in carrying the Iceni as far south as the frontiers of Essex and Herts. (Ann. xiv. 31—37.)
The real reason, however, for fixing the Iceni lies in the assumption that they are the same as the Simeni of Ptolemy, whose town was Venta (Norwich or Caistor): an assumption that is quite reasonable, since the Venta of Ptolemy's Simeni is mentioned in the Itinerary as the Venta Icenorum, and in contradistinction to the Venta Belgarum (Winchester).