Septimius Severus came to Britain in 208 AD, a sixty-two-year-old emperor in poor health, accompanied by his sons Caracalla and Geta. His objective was to subdue Caledonia - the territory of the Picts and their predecessors - and add it to the empire that already stretched from Mesopotamia to the Pillars of Hercules. The campaign was vast and brutal. Severus marched legions deep into the Highlands and back. He took heavy losses to weather and guerrilla warfare more than to pitched battle. Carpow served as a naval supply depot for the operation, a forward base where galleys ascending the Tay estuary could unload grain, weapons, and reinforcements for armies pushing inland. The fort's occupation precisely coincides with the Severan campaigns, and it was abandoned not long after Severus's death at York in 211 AD.

What sets Carpow apart from the dozens of temporary Roman camps that dot the Scottish landscape is that it was built for permanence. Most Roman incursions into Caledonia left only earthworks - ditches and timber palisades that fell apart within a generation. Carpow was built of stone, brick, and tile. It was occupied at different times by Legio II Augusta and Legio VI Victrix, two of the most senior units in the imperial army. II Augusta had been part of the original invasion of Britain in 43 AD and was based at Caerleon in South Wales. VI Victrix was the garrison legion of York. Bringing them this far north was a major commitment of imperial resources. Carpow was meant to last.

An eighth-century Byzantine geographical text called the Ravenna Cosmography names a place in eastern lowland Scotland called either Horrea Classis or Poreo Classis. Horrea Classis translates from Latin as 'Naval Storehouses,' which fits Carpow's role as a supply depot on a tidal estuary almost perfectly. Poreo Classis makes no obvious sense and is generally treated as a scribal error - though some scholars suspect it preserves the Latinised form of a pre-Roman Brittonic place name, possibly explaining the survival of the element 'Pow' in the modern name Carpow. The identification with Horrea Classis is widely accepted but not certain. What is certain is that this was a major naval supply installation - a Roman port at the head of the Tay estuary.

The fort has never been comprehensively excavated. Aerial photography and limited digs have revealed the outline of a roughly twenty-seven-acre rectangular enclosure with the standard Roman layout: a principia at the centre, granaries to support the garrison, barracks for the legionaries, and defensive walls. Stamped tiles and inscriptions on masonry have confirmed the presence of both legions. But most of the fortress still lies beneath a Perthshire field, waiting. The Bronze Age log boat discovered nearby in 2001, dating from around 1000 BC and now displayed at Perth Museum, is a reminder that the Tay had been a major waterway for at least a thousand years before the Romans arrived to build their supply base. When Severus's legions sailed up the estuary in 208 AD, they were following a route that had been carrying boats for forty generations already.