Chester was founded as a "castrum" or Roman fort with the name Deva Victrix in the reign of the Emperor Vespasian in 79 AD. One of the main army camps in Roman Britain, Deva later became a major civilian settlement. In 689, King Æthelred of Mercia founded the Minster Church of West Mercia, which later became Chester's first cathedral, and the Saxons extended and strengthened the walls to protect the city against the Danes. Chester was one of the last cities in England to fall to the Normans. William the Conqueror ordered the construction of a castle, to dominate the town and the nearby Welsh border.
Chester is one of the best preserved walled cities in Britain. It has a number of medieval buildings, but some of the black-and-white buildings within the city centre are Victorian restorations. Apart from a 100-metre (330 ft) section, the listed Grade I walls are almost complete. The Industrial Revolution brought railways, canals, and new roads to the city, which saw substantial expansion and development – Chester Town Hall and the Grosvenor Museum are examples of Victorian architecture from this period.
The Roman Legio II Adiutrix during the reign of the Emperor Vespasian founded Chester in AD 79, as a "castrum" or Roman fort with the name Deva Victrix. It was established in the land of the Celtic Cornovii, according to ancient cartographer Ptolemy, as a fortress during the Roman expansion northward, and was named Deva either after the goddess of the Dee, or directly from the British name for the river. The 'victrix' part of the name was taken from the title of the Legio XX Valeria Victrix which was based at Deva. Central Chester's four main roads, Eastgate, Northgate, Watergate and Bridgegate, follow routes laid out at this time.
A civilian settlement grew around the military base, probably originating from trade with the fortress. The fortress was 20% larger than other fortresses in the Roman province of Britannia built around the same time at York (Eboracum) and Caerleon (Isca Augusta); this has led to the suggestion that the fortress, rather than London (Londinium), was intended to become the capital of the Roman province of Britannia Superior. The civilian amphitheatre, which was built in the 1st century, could seat between 8,000 and 10,000 people. It is the largest known military amphitheatre in Britain, and is also a Scheduled Monument. The Minerva Shrine in the Roman quarry is the only rock cut Roman shrine still in situ in Britain.
The fortress was garrisoned by the legion until at least the late 4th century. Although the army had abandoned the fortress by 410 when the Romans retreated from Britannia, the Romano-British civilian settlement continued (probably with some Roman veterans staying behind with their wives and children) and its occupants probably continued to use the fortress and its defences as protection from raiders from the Irish Sea.