AD 324

Constantinople

Constantine begins refounding Byzantium as a new imperial capital

The new walls enclosed more than three times the area of old Byzantium — before there were anywhere near enough houses or inhabitants to fill it.

November 324, weeks after Chrysopolis

Constantine defeated Licinius at Chrysopolis on 18 September 324 and became sole emperor. Within weeks he had selected the existing Greek city of Byzantium for a new imperial and dynastic capital, and construction began there in November.

No secure contemporary source gives a universally accepted groundbreaking day. Treccani treats 18 November — the day Constantius II was elevated Caesar — as the probable date of the legal delimitation that fixed the expanded urban boundary; other later traditions give 8 November.

An expansion, not a new town

The project was a monumental refoundation of a city that already existed, not a settlement raised on empty ground. It aimed to assimilate the new capital institutionally and ceremonially to Rome while creating a political centre for the eastern part of a still-unified empire.

Later accounts describe Constantine enlarging Byzantium, ringing it with new walls, and providing the buildings, population and Senate expected of an imperial capital. The expanded Hippodrome reached roughly 450 metres.

No Byzantine Empire was involved

There was no "Byzantine Empire" in 324. That is a modern name for the medieval Roman state whose inhabitants called themselves Romans, and what Constantine had defeated was a co-emperor, not a foreign power.

The city was not finished quickly either. Building continued for about six years, until the formal dedication of Constantinople as New Rome on 11 May 330.

Sources

Researched 23 Aug 2026 8 sources not yet audited Date corrected

How this was checked

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Date corrected. The research places this at 0324-11.

What the sources leave uncertain

The month is well supported, but no secure contemporary source supplies one universally accepted groundbreaking day.
Treccani considers 18 November—the day Constantius II was elevated Caesar—the probable date of the legal limitatio that established the expanded urban boundary. Other later traditions give 8 November.
Ancient and medieval accounts embellish Constantine’s selection and surveying of the site with divine guidance; these stories are not independently verifiable.
Constantine’s military acquisition of Licinian Byzantium, his later choice of it for a new capital and the start of construction were related but distinct stages.