CONSTANTINE cut a striking figure. "In handsome physique and bodily height he so exceeded his contemporaries as even to put them in fear," says one gushing account of the Roman emperor.

"He took pride in moral qualities rather than physical superiority, ennobling his soul first and foremost with self-control, and thereafter distinguishing himself by the excellence of the rhetorical education, his instinctive shrewdness and his God-given wisdom."

That eulogy came courtesy of Eusebius, a bishop, who clearly knew on which side his bread was buttered.

Yet history has proved the biographer right: Constantine was an extraordinary man whose remarkable influence on the world resonates to this day. And he is inextricably linked to York.

This Friday, York Museums Trust opens to the public a major exhibition dedicated to Constantine, "York's Roman Emperor". It runs at the Yorkshire Museum until the end of October to mark the 1,700th anniversary of him being declared emperor in our city, on July 25, 306.

Considering how many centuries have passed since that day, we know an impressive amount about the man. Born in about 272, He came from good stock: his dad, Constantius became one of the four rulers of the Roman Empire in 293.

Mum Helena came from a humbler background, but became a saint after being credited with finding the relics of the true cross. She has an island and St Helen's Church and Square in York named after her.

Constantine rose to power through the Roman army and some nifty networking, which included marrying the daughter of another emperor, Maximian.

In 305 dad Constantius led his second campaign in Britain, and brought Constantine with him. Eboracum, as York was then known, was one of the regional capitals in the Roman province of Britannia. Its legionary fortress was the headquarters of the northern military command.

The fortress stood on the site where York Minster stands today and visitors can still see a massive Roman column, which was found by archaeologists underneath the Minster Tower. The column had been one of 16 supporting the architrave of the fortress headquarters hall.

When the emperor was in Britain mounting campaigns in the north, York also became the seat of the Imperial Court. The palace was located in a different part of the city.

And it was most likely here where Constantine was declared emperor on the death of his dad.

"The deathbed scene over, Constantine emerged from the palace wearing the purple imperial robe and personally led the funeral procession, hailed by troops and people with the imperial title Augustus," writes Averil Cameron, in the book which accompanies the new exhibition.

A good start, but the ruler is still not "great". There were three reasons why he earned that epithet, according to Roman expert Dr Christopher Kelly, of Cambridge University.

Reason one: after nearly 80 years, and three generations of political fragmentation, Constantine united the whole of the Roman Empire under one ruler. By 324 he had extended his power and was sole emperor, restoring stability and security to the Roman world.

Reason two: Constantine abandoned Rome as the most important city in the empire, building a new capital modestly named Constantinople (now modern Istanbul). In the next two centuries, Rome and Italy became vulnerable to barbarian invasions. The much more easily defensible Constantinople lasted for another thousand years.

Reason three: Constantine's strong support for Christianity arguably had the greatest impact on European history. He is said to have been converted to the faith in AD 312.

At the time, only around ten per cent of the Roman empire's population was Christian. The majority of the wealthy, ruling elite worshipped the old gods of Rome. Constantine was responsible for adopting, uniting and promoting Christianity.

He was a giant figure, says the Very Rev Keith Jones, the Dean of York. "It was because Constantine decided to support the Christian Church as the key institution of the Roman Empire that Christianity triumphed."

In her introduction to Constantine The Great: York's Roman Emperor, Elizabeth Hartley, writes that about the impact of the man on the city.

At that time York was the capital of one of four provinces which made up the Diocese of the Britains.

"It thus seems no accident that the larger-than-life-size marble head of Constantine should have been discovered in York in the area of the fortress near the headquarters building. The hall of the headquarters building was rebuilt under Constantius or Constantine and a statue of Constantine is likely to have been placed there. There are also still standing the monumental remains of the corner-tower of the fortress with its connecting curtain wall, which fronted the river.

"The magnificence and sheer scale of this frontage when complete suggest that it was built with imperial backing, suitable for the high status of York and its position as a place where emperors resided and major events took place."

Constantine didn't stay long in the city, establishing Trier as his base for his campaigns against the Germans perhaps a year after his succession. But that move did not end his association with York.

"Although we lack details of Constantine's visits to Britain after his initial accession, coin evidence does suggest that he was there on more than one occasion during his first ten years in power," writes Elizabeth Hartley.

"Almost certainly, York would have been one of the places to experience again an imperial adventus (arrival) and period of residence of emperor and court."