Byline: Stephen Lewis

NEXT time you're walking along Rougier Street, it might be worth pausing for a moment to think about what lies beneath. Because chances are that you might just be following in the footsteps of a Roman emperor or two.

Somewhere hereabouts there may well once have a been a Roman Imperial palace.

We don't know exactly where, admits Ian Milsted of the York Archaeological Trust.

But at least two Roman emperors once based themselves in York for a while: the Emperor Septimius Severus from 208-211 AD, and the Emperor Constantine, who was almost certainly proclaimed emperor here in 306 AD.

York then was at the very farthest northern edge of empire, Ian says. "It was 'here be dragons' territory. But if the emperor came here, and spent time here, there has to have been somewhere for him to live."

Severus based himself here while leading campaigns against the Caledonians north of Hadrian's wall, who had been attacking targets in the north of Roman Britain. But while he was here the vast Roman empire was effectively governed from York - or Eboracum, as the Romans knew it.

Severus, known as the 'African Emperor' because he had been born in Libya, is though to have brought a huge retinue of civil servants and soldiers with him.

But if there was an 'imperial palace' to house the emperor and his retinue, where was it? The simple answer is that we don't know, Ian admits: except that it wouldn't have been too close to the river, and would likely have been on slightly higher ground.

So what do we know about Roman York?

The Roman Ninth Legion, an army unit of about 5,200 men, arrived here in 71 AD and founded a fortress. Fifty years later, in about 120 AD, it was replaced by the Sixth Legion, which helped with the building of Hadrian's Wall.

The fortress remained an important army base until the Romans left York in about 410 AD. And at some point...