It is famed as the place where gladiators battled lions to the amusement of the citizens of ancient Rome.

It even sparked an Oscar-winning film.

But of the six million visitors who flock to the Colosseum each year, how many know that during mediaeval times, it was also the fortress base of a powerful Roman family for over two centuries?

Or that in the 1600s, it was like a  botanical garden? Its state of semi-abandon, combined with a micro-climate, enabled more than 400 species of plants to flourish inside its arched walls.

Now, thanks to some archaeological detective work based on discoveries made during a spruce-up, a new exhibition recounts some of the untold stories of one of the world's most-visited monuments.

'The Colosseum: An icon', which opens on a middle floor of the amphitheatre this week and runs until next January, shows that life inside the iconic structure did not end with the disintegration of the empire or the final show of the classical era, in 523 CE.

Scholars had long been aware that the mediaeval-era Colosseum had a fortress owned by the Frangipane family.

But much of the archaeological evidence of it was lost at the time of 19th-century excavations, when masonry was removed for new buildings or restorations elsewhere in the city.

Recent restoration work on upper sections of the partially-intact structure however uncovered traces of what was a raised wooden walkway which served as a lookout for the Frangipanes' soldiers, constantly wary of attacks by rival families.

Other archaeological finds, including one side of a ram's head and carved antlers, point to the mediaeval Colosseum being a hive of activity with the fortified aristocratic residence serviced by a range of businesses, market gardens and religious institutions.

The wooden fortress was partially destroyed by a 1349 earthquake but its surviving structures were later incorporated into a hospital sponsored by wealthy families whose seals have been recently found in digs on the site.

Completed in 80 CE, the Colosseum was the biggest amphitheatre built during the Roman empire.

Standing 48.5 metres (159 feet) high, it was capable of hosting 80,000 spectators for feasts of entertainment that encapsulated the brutality, hedonism and engineering genius that were among the defining features of ancient Rome.