The Forum of Constantine was circular in plan — ancient sources use both oval and circular to describe it — with two monumental gates, one to the east and one to the west, marking the transit of the Mese through the square. It was the city's central reference point: distances were measured from here, processions organized around it, and the Senate House, the city's first, occupied the forum's northern edge. The scale was deliberate. Constantine was founding not just a new city but a new Rome, and the forum performed that ambition in stone. Antique statues from across the Greek world were brought to decorate it — the sources note them without being specific enough to allow a precise catalogue, though their loss in 1204 means the question is largely moot. What the forum looked like at its peak, in the early fourth century, can only be reconstructed from the architectural logic of what remained and the written testimony of those who described it.

At the forum's center, Constantine raised a column of purple porphyry — the imperial stone, quarried in Egypt and used almost exclusively for imperial purposes. On the column's summit stood a statue of Constantine himself, rendered as Apollo, with rays emanating from his head. The column carried the emperor's image into the sky above the city he had built. In 1106, a violent storm brought the statue and three of the column's upper drums crashing down, killing around ten people. The Byzantine emperor Manuel I Komnenos replaced the statue with a cross and repaired what could be repaired. The column survived earthquakes, fires, and conquest. Today it stands in the neighborhood now called Çemberlitaş — the name means 'hooped stone' in Turkish, referring to the iron bands fitted around the drum sections to keep them together — still rising above the street, fire-darkened from a conflagration in the column's long history, still visited daily by people who may or may not know what they are looking at.

The Forum of Constantine remained largely intact through Byzantine history until the catastrophe of 1203–1204. The Fourth Crusade, diverted from its stated purpose of reaching the Holy Land, arrived at Constantinople in the summer of 1203. In the fighting that followed, a fire set by crusading soldiers swept through the city and struck the forum, causing major structural damage. The following year, when the crusaders sacked the city outright, the wholesale destruction of Constantinople's ancient heritage accelerated. The antique statues that had decorated the forum — brought from the Greek world, assembled as a visual library of classical civilization — were melted down. Bronze for coin and weapon; marble left to rubble. The particular cruelty of that loss is that these were statues preserved from the ancient world precisely because Constantinople had collected and protected them. They survived a millennium of Byzantine history and did not survive the Crusaders.

After the sack of 1204, the forum's role as the city's ceremonial center diminished without ever quite disappearing. The Latin Empire that ruled Constantinople for the following decades used the city but invested little in it. When the Byzantines retook Constantinople in 1261, the forum had been damaged and depleted. Under Ottoman rule after 1453, the area around the Column of Constantine — now called Çemberlitaş — remained a significant urban node, and the surrounding neighborhood took on the column's popular name. Today, the Çemberlitaş Hamamı, one of Istanbul's historic bathhouses designed by the great Ottoman architect Mimar Sinan in 1584, stands nearby. The old forum's footprint has been absorbed entirely into the urban grid of the historic peninsula, but the column at its center keeps marking the place where Constantine imagined the world's center to be.