After the Barbarian invasions that triggered and followed the demise of the Western Roman Empire the population of Florence fell from an estimated 10,000 to as low as 1,000. The Eastern Roman Empire (later known as the Byzantine Empire) began the process of reclaiming Italy after the year 500. When they arrived in Florence it was a sad picture that they found: the city was reduced and new walls to give better protection to it had to be built. Because of few remains and scarce historical records, some historians even doubt these walls ever existed. It is believed the Torre della Pagliazza, now a part of a hotel, could have been part of this second wall. It is certainly very old, perhaps the oldest building in Florence, dating from the VI or VII century, built either as part of the new walls or perhaps by the Lombards as a private house. It was surely erected on Roman foundations and is unique in Florentine architecture, being the only round structure in the whole city centre. Excavation has brought to light part of the original pavement under the tower, but nothing relating to the Byzantine wall has been found.

In the X century work begun on a third wall around the city that also followed the line of the Roman wall, though a bit wider, to include the area around the Battistero and the Church of Santa Reparata (where now the Cathedral stands) and arriving almost to the Arno, where a port was built.

During the period that Tuscany was under the marquises, notably under Mathilde, a fourth wall was built, reinforcing the pre-existent one with the sole addition of a castle on the waterfront, the Castello di Altafronte, which stands on the site of the present Museo di Storia della Scienza, almost next door to the Uffizi.

Between 1173 and 1175 a fifth wall was built by the Florentines, this time including the broghi, that is, the roads that led from the outside to the each of the gates in the fourth wall. Many of these broghi still exist and some have the very same names. Not all of thesem date from this period, but at one point or another, all of these were outside the walls. I can cite Borgo Pinti, Borgo San Jacopo, Borgo Ognissanti, Borgo la Noce, Borgo Tegolaio, Borgo La Croce, Borgo degli Albizi, Borgo Allegri, Borgo Santissimi Apostoli, Borgo Santa Croce, Borgo San Frediano and Borgo dei Greci. This new wall crossed the river for the first time, including part of the Oltrarno, as well as, on the north bank, the Church of San Lorenzo. It is believed the Arco di San Pierino (Volta di San Piero), at the beginning of Borgo Pinti was one of the gates in this new wall.

Starting in 1282 a sixth wall was built around Florence. Among its architects figures the name of Arnolfo di Cambio, though this is not attested in any document and in any case he would not have been the sole architect designing them.  The wall had 63 watch towers as well as 12 gates. While on the north side they were demolished in the XIX century, only some of the doors and watch-towers been preserved, the south wall, apart from a small section which has been demolished, is almost intact. With one exception all doors were reduced in height as suggested by Michelangelo as a defensive measure against artillery fire. Among the surviving gates on the north bank are Porta La Croce (Piazza Beccaria), Porta San Gallo (Piazza della Libertà) and Porta al Prato, while Porta Faenza has been incorporated into the Fortezza da Basso. On the south bank the only isolated door is Porta San Niccolò, in Piazza Poggi, just under Piazzale Michelangelo, which is the only city gate not to have been shortened; consequently being of the same height the doors were originally.

The most significant feature of Florence is without doubt the cathedral complex, consisting of the Cathedral of Santa Maria del Fiore, plus the baptistery of St. John (Battistero di San Giovanni) and Giotto’s Belltower (Campanile di Giotto). Together with the Baptistery and the tower, the cathedral is the emotional, if not the physical centre of Florence while the dome of the Cathedral is the tallest and most visible of all the architectural monuments of the city.

The oldest of these is the Baptistery. Work on a first building on an octagonal plan, apparently on the remains of a guard tower on the Roman wall, began in the late 300s or early 400s. A second one, on the same plan, but larger, was constructed by the Lombards in the VI century, though before 897 no records of its existence survive. A third, much enlarged baptistery was began in the mid XI century and consecrated in 1059. A lantern was added in the following century, while a rectangular apse was added in the early XII century, while three sets of bronze doors were installed, starting in the XIV century.

The Cathedral of Santa Maria del Fiore was built over the site of the earlier cathedral, Santa Reparata, which was not immediately demolished, but rather incorporated into the new building and remained in use till its final demise in 1375. Construction began in 1296 under Arnolfo di Cambio. Work continued over the centuries under the direction of several architects, among them Francesco Talenti and Giovanni di Lapo Ghini. By 1421 the main body of the cathedral was ready, but there remained the dome to be constructed. This had posed a problem since 1315, when the drum for the cupola was ready, but, as non one was capable of designing a cupola that would not collapse, it had been left as it was.

Giotto’s Belltower (Campanile di Giotto), a free-standing structure, was projected by the painter Giotto di Bondone from 1334, though he did not live to finish his work, being succeeded first by Andrea Pisano and, after by Francesco Talenti . It is a magnificent example of the Gothic style in Florence, faced with white, pink and green marble and decorated with statues as low-reliefs.

From the XI and XIII century dates the tower house, a type of ancient skyscraper which was usually built by a powerful family as a refuge during times of civil strife. A considerable number of these have disappeared: some were demolished in a bid to curtail the civil wars which broke out periodically between the several families; some were demolished when a family backed the wrong political faction and was consequently exiled, while still others were incorporated into new buildings. Some actually managed to survive into modern times in the vicinity to the Mercato Vecchio, only to be torn down in the XIX century. Surprisingly enough, there still are many of these twoer houses still in existence. They are tall, grim, square and narrow and made of stone with holes on the masonry which were used in olden days to place beams for scaffolding and to build bridges between neighbouring towers.  Among the extant ones the best-known is the Torre dei Manelli, at the Oltrarno end of Ponte Vechhio, being skirted by the Corridoio Vasariano. This detour was made necessary, as the Manelli family refused to sell the tower to the Medici or to allow the passageway to go through their property, as other families had. Other Towers are the Torre degli Albizi, Torre dei Ramaglianti, Torre della Castagna, Torre degli Alberti (this latter one has a different design, in that it is not rectangular, but a trapeze), Torre dei Belfredelli, Torre dei Visdomini, Torre dei Peruzzi, Torre dei Pierozzi, Torre dei Ricci, Torre dei Ghiberti, Torre dei Donati, Torri dei Galigai, Torre dei Della Bella, Torre dei Cerchi, Torre degli Alepri and Torre dei Sacchetti.

Some of the oldest churches in Florence, as well as many tower houses situated in the area around the Mercato Vecchio, were demolished during the second half of the XIX century, including The Church of San Miniato tra le Torri, on the site where it is believed San Miniato was martyred.

The Church of Santa Maria Sopra Porta, so called because it stood near the old Porta Santa Maria, dates from the beginning of the XI century. It has been deconsecrated and is now the library of the Palagio di Parte Guelfa. It is the last of the Churches in the vicinity of the Mercato Vecchio to preserve an external flight of steps, as was the norm in all other minor churches in the vicinity.
The Palagio itself, Palagio dei Capitani di Parte Guelfa, was built in the XIV century to house the Guelph faction during the troubles with their rivals, the Ghibellines. The word palagio, from where Modern Italian Palazzo, refers to any Florentine building larger than a tower house, but not yet the family palace of the Renaissance.

Another very old church is San Remigio, considered by some as the oldest that still maintains some of its original appearance. Dating from the XI century (though a plaque outside the church claims it to have been existent in the year 800) is the Church of the Santissimi Apostoli, in Piazza del Limbo. Restoration has given it again its Romanesque appearance.

The Badia Fiorentina was founded by Willa of Tuscany, mother of Marquis Hugh, in 978. It was subsequently altered by Arnolfo di Cambio, the architect of the cathedral. The bell tower dates from the XIV century. The Badia stands across the street from the Bargello, a castle, former city hall, house of the podestà (a type of mayor), palace of the Bargello (a public official with policing functions) and prison dating from the XIII-XIV centuries. It now houses the Museo Nazionale del Bargello, the national sculpture museum. It keeps most of its appearance as a fortress, complete with battlements and a watch tower, where a bell is situated, to be rung only in times of peril.

The tract if the old decumanus maximus connecting Piazza della Signoria to Piazza del Duomo ( enlarged in the XVI century). On this street, facing each other, are two old churches: Orsanmichele or San Michele in Orto and San Carlo dei Lombardi. The former has a curious shape for a church, in that it is a square building with three floors. This is easily explained, as originally it was not a church, but a loggia where a grain market was situated. This was built on the site where until 1240 stood the church of San Michele Arcangelo, demolished to make place for the market. A first loggia is believed to have been built by Arnolfo di Cambio in 1290. This burned down in 1304 and a new loggia was created by, among other, Francesco Talenti, one of the architects of the Duomo. Statues were commissioned by the guilds for the niches in the external part of the columns. In 1380, after some miracles were observed at the loggia, the market was moved, the arches of the loggia closed and a church was consecrated instead, while an extra two floors were built over the church to serve as a granary. After changing function during in the time of Cosimo I, these top floors now house a small museum which can be reached through a passageway from the Palagio di Parte Guelfa. The church is characteristic for its Gothic windows and the statues, on the façades, of the patron saints of the Florentine guilds. Inside the most notable feature, a church within a church as it were, is a tabernacle by Andrea Orcagna. There are vestiges of the original paintwork on the vaults. Orsanmichele is also remarkable in that it is one of the few free-standing buildings in the centre of Florence, having a façade on all four sides.