We were looking at an intaglio - a gemstone from a Roman's finger-ring. I was even more amazed that we could see it had been beautifully engraved with a Capricorn, a cornucopia (horn of plenty), a tiny fish and what looked for all the world like an eagle!

The found intaglio

I was even more amazed that we could see it had been beautifully engraved with a Capricorn, a cornucopia (horn of plenty), a tiny fish and what looked for all the world like an eagle!

Caerleon, the site of the legionary fortress known to the Romans as Isca, was the headquarters of the Second Augustan Legion from A.D.74/75 until about 300. The Second Augusta, a unit of some 5,000 heavily armed and lethally efficient soldiers, had been part of the invasion force in 43 and was involved in fierce fighting throughout southern Britain, including the final campaigns against the fierce Celtic tribes of what is now South Wales. We were in Caerleon carrying out key-hole excavations to evaluate the archaeological remains of the newly discovered suburb of massive Roman buildings that Tim Young and his teams of undergraduates had discovered on their geophysics training courses in 2010.

The geophysical results of the new suburb are remarkable and show the remains of several very large public-style buildings beneath green fields outside Caerleon, including one of the biggest structures known from Roman Britain that we believed was the fortress port. Although Caerleon has been known about for a long time - large-scale excavations were carried out across the fortress during much of the 20th century - the landscape outside Isca's gates was almost entirely unknown. The discovery of the port and associated suburb was a huge breakthrough in our understanding of the site. This discovery suggests that we have to see Isca as much more than a legionary fortress. Now we need to think about its role as the place from which the army fighting in Wales was supplied and a major port in pacified Britannia. If you want to know about Roman Britain you have to understand the Roman army - after all there wouldn't have been a Britannia without legionaries such as those garrisoned and living in Isca.

To find out what the suburbs means requires digging to recover evidence that will tell us when the buildings were constructed and what they were for. That's why archaeologists from Cardiff University asked Time Team to join them this year, and I'm pleased we did because it was a great opportunity to film some of the most remarkable Roman archaeology in Britain that I have seen for a long time.

We opened 8 long narrow trenches across and area of about 5 hectares between Caerleon's famous amphitheatre and the River Usk on which Isca was strategically positioned. We had been digging for 3 weeks when Time Team opened a 9th trench over an intriguing rectangular geophysical anomaly in the courtyard attached to the port facilities. While Phil and his diggers were cleaning Trench 3 so we could see what was going on, Rakshar, Matt and Alex joined teams in the open trenches and immediately began working in Roman rooms with walls standing to waist height. The trenches contained painted plaster still in situ, exposed lead water-pipes - perhaps taking fresh water to drinking fountain or troughs elsewhere in the complex, underfloor heating systems and hundreds of finds, each of which told us about Caerleon's ancient history. One fragment of stone from the port trench was part of a monumental inscription. Although only 3 letters survived I was amazed to read AVG - an abbreviation of AVGVSTVS ('emperor' or 'of the emperor') or AVGVSTA (as in 'Second Augustan Legion'). This confirmed that the port, and probably the entire suburb, was an official imperial foundation almost certainly dating to the early years of the Roman conquest and subjugation of the native people of western Britain. We were learning about the importance of the site in the creation of Britannia.

I was continually surprised at how well remains survive and how shallow they lie under the grass - where was the medieval occupation that usually covers, and often damages, the Roman archaeology? We were finding Roman floors only a few centimetres below the modern ground level - what had happened here so that these enormous buildings had disappeared and this part of Isca been forgotten about? It turns out that the answer lies in the later Roman period when the suburb appears to have been abandoned and been used as a dumping ground for rubbish, before being systematically robbed and most of the stone taken away. Unlike other parts of Caerleon where Medieval chroniclers saw standing Roman buildings, our suburb was deliberately dismantled probably for its stone which was needed elsewhere on the Severn Estuary. It was amazing to find the lead-water pipe in Trench 6 and only slightly frustrating that we couldn't follow it to discover where it came from and where it was headed to!

In the end there wasn't a lot we could say about the enigmatic building in Phil's Trench 3, though it is very unusual as it had been built entirely from brick rather than stone and mortar. On the other hand T3 did produce the intaglio, which has to be one of the most fantastic objects I have ever held in my hand. The Capricorn was the birth sign of Augustus, the first Roman emperor who raised the Second Legion bearing his name many years before the invasion of Britain. It was also, therefore, the emblem of the Legion itself so it has great significance in Caerleon and the fact that it is combined with the cornucopia, the symbol of abundance, makes you think of the message conveyed by this tiny object - Augustus and the legion bringing prosperity for all? The eagle seemed to be too good to be true - after all, what better symbol could there be of Roman imperial power extending to exotic places like South Wales? Sadly, it was too good to be true and the eagle turns out to be a cockerel, symbolising the god Mercury! Never mind, one of the great things about archaeology is that you never stop learning things you never knew about objects, people and places. A dig is all about stories, old and new. Our dig with Time Team was just the latest chapter in the long history of Caerleon, but it was a great one!

Find out more about the Lost Roman Port and a variety of Cardiff University Projects at Caerleon