The ancient Roman city, most probably started as a military camp (castrum). It was definitively carefully laid out, following the grid plan used for all Roman settlements, with two main roads crossing the entire settlement from gate to gate: the cardus maximus, running north-south and the decumanus maximus, running east-west. Where these crossed was the forum and in the centre of this forum was a column marking the exact centre of the city. It was from this column that all distances from Florence were measured. Around the ancient city there was a wall, with five gates, two at either end of the cardus maximus, two othera at either end of the decumanus maximus and one at the end of the present Via Porta Rossa. There is almost no visible trace of this first Roman town, but its design has been preserved in the modern road plan of the city centre and is clearly visible on any city map.

The forum corresponds roughly to the modern Piazza della Repubblica. The ancient column has gone, substituted by the Colonna dell’Abbondanza in 1431. Originally there was a statue of Abundance by Donatello placed on the top, but this eventually crumbled and fell in the XVII century, being substituted by another on the same subject by Giovan Battista Foggini. In the Mediaeval city and up to a major refurbishment in the XIX century, the ancient forum was used as a market, being in consequence known as the Piazza del Mercato (and after the construction of a new market, just down the street as Piazza del Mercato Vecchio). Soon after the unification of Italy and in order to give to Florence, for a brief period the new capital of the kingdom, a more modern aspect, the old market (together with the nearby ghetto) was demolished and renamed in honour of the king as Piazza Vittorio Emmanuele II.  The column was moved to make place for a statue of the monarch and it was only in 1956 that it came back again not to mark the real centre of modern Florence, but the centre of the ancient settlement. Foggini’s statue was substituted by a copy, while the original is now in the custody of the Cassa di Risparmio di Firenze.

The cardus maximus, corresponds now to three roads: to the north and leading to Piazza della Repubblica we have Via Roma and to the south, leading from the piazza to Ponte Vecchio, Via Calimala and Via Por Santa Maria. The decumanus maximus, the east-west road, has become to the west of the forum, Via degli Strozzi, while to the east it has been divided into two roads leading from the piazza: Via degli Speziali and Il Corso (sometimes erroneously called Via del Corso).

The walls are no longer extant, but instead, to east, west and south there are roads that run over their foundations: to the east Via del Proconsolo, to the west Via dei Tornabuoni. To north the line of the wall is not quite as apparent, but part of Via dei Banchi and Via dei Cerretani do run along part of the wall, which continued where now Piazza del Duomo is. To the south there is no road along the line of the ancient wall, but its site corresponds to the southern end of Piazza della Signoria. When road works were undertaken in Via del Proconsolo the foundations of one of the Roman towers was found. It obviously could not be left exposed; so instead an almost complete ring was made on the surface the road, using metal and stone, allowing passers-by to see the location of this tower. Nearby there is an explanatory plaque, with designs of how this wall might have been. Continuing along Via del Proconsolo,at the corner with the Corso there is now a modern construction where before a Mediaeval building used to be, demolished for some reason which is not clear to me. The ground floor is built entirely of glass; walls and floor, and under this glass floor the curious passer-by can see the foundations of one of the gate towers as well as a section of the Roman wall.

Some of Ancient Florence’s secondary roads running parallel to the cardus maximus and to the decumanus maximus are still there, albeit with different names. Running east-west, parallel to the decumanus maximus, we have the following axes: Via dei Pecori-Via degli Agli, Via delle Oche-Via dei Tosinghi-Via del Campidoglio,Via della Condotta-Via Porta Rossa and Via dei Lamberti-Via dei Cimatori; while running north-south, parallel to the cardo maximus, we have Via dei Pescioni-Piazza Strozzi-Via Monalda, Via dei Vecchietti-Via dei Sassetti, Via dei Brunelleschi-Via Pelliceria, Via dei Calzaiuoli, Via Santa Elisabetta-Via dei Cerchi and Via dei Magazzini-Via dello Studio. These latter no longer connect, however.

Inside the city gates there were also baths and a theatre, the latter located roughly where Palazzo Vecchio now stands. Archaeological excavation within the perimeter of the ancient city has brought many remains of the Ancient city to light, principally in Piazza della Signoria, where there is an explanatory plaque showing the layout of Ancient Florence. Some streets have even kept the memory of the Roman town, , for example: Via delle Terme, and Via di Capuccio (from Latin caput acquae, that is, origin of the water) take their name from the nearby baths, and Via del Campidoglio, near the forum, where the Capitol was found. At Via delle Terme there is a building that claims to be the site of the ancient baths, as a phrase over the door proudly announces.

To the south-east, outside the city walls, there was an amphitheatre, a sort of mini Colosseum, where games were held, as they were in Rome. It was located between the modern Piazza Peruzzi, Via Torta and Via dei Bentaccordi and it is corossed by Via dell’Anguillara and Borgo dei Greci. The amphitheatre has also disappeared, but the site where it was located is clearly visible, as all later buildings were erected on its foundations. One of the roads where it was located is now aptly named Via Torta, that is, crooked street.

There are two streets that commemorate city gates in this ancient Roman wall: Via Por Santa Maria, that is: Via di Porta Santa Maria, and Via Porta Rossa.

The southern walls were extended at a later date, to include the theatre (not to be confused with the amphitheatre!) and the baths. It now ran parallel to Borgo Santissimi Apostoli and Via Lambertesca.

To the northeast of Florence, just outside this first circle of walls, there was a sacred zone, where it was believed that in more ancient times some pagan temples were situated. After the conversion of the Roman Empire to Christianity, a sacred axis was established, comprising Palazzo Vescovile, il the Battistero, a hospital, a rectory, a cemetery, and the Churches of  San Salvatore al Vescovo, San Michele Visdomini and Santa Reparata. The latter, the predecessor of the modern cathedral, is believed to have been built in the VI century, followed by the Battistero, though none of this is certain, as the latter is believed to have been founded between the IV and V centuries . The foundations of Santa Reparata, which can be visited in the crypt of the Duomo, attest in fact to the Roman nature of this building and the architectural style of this battistero, a hexagon, cannot be fully understood unless we can relate it to Ancient Rome.

There are claims that the Battistero, in Piazza san Giovanni, was in origin a Roman temple dedicated to Mars, but there is no evidence for this and the present building, though very ancient indeed, is from a later date, build after the old Cathedral of Santa Reparata was founded in the VI century. There are, however, vestiges of an old Roman floor under the Battistero, but these are clearly unrelated to the modern building.

The Church of Santa Reparata was originally situated just outside the city walls, almost precisely where the modern cathedral now stands. There is little idea of what it might have looked like, but it is know it had two bell towers. It is thought to have been older than the Battistero. Under the cathedral there are vestiges of this church, such as the foundations and the pavement, which can be visited. It began to be pulled down in the XIII century, to make place for a new cathedral, more in line with the pretensions of a larger richer city. There is no precise date when Santa Reparata was built, but tradition seems to place its foundation in the V century, before the fall of the Western Roman Empire.

Also outside the city walls, to the northwest, stood the first cathedral of Florence: San Lorenzo, consecrated in 393. Though this church still exists in the same location and is one of the most important churches of Florence, there are virtually no remains of this ancient cathedral in the modern building.

Other vestiges of the Roman city are to be found in the columns incorporated into other buildings. Some can be found, for example, in the Church of San Miniato al Monte, where they are clearly identified.

Estimates make the population of Roman Florence to be of around 10,000 people.