The subdivision of the terra nuova (“new town”) was designed by the Florentine Republic to provide an orderly settlement for the six rural communities from the surrounding countryside who were to become the first inhabitants of Castel Santa Maria (later the community of Terranuova): the communities of Mori, Ganghereto, Pozzo, Terraio, Pernina, and Cave. The populations who moved there at the time of the foundation brought with them their local and religious traditions, rebuilt within the walls the churches of the villages they had left behind, and kept the original place names in their dedications.
This allowed each group to preserve its own identity within its quarter and around its church. Thus were built the churches of San Niccolò at Ganghereto, San Bartolomeo at Pozzo, Santa Maria at Pernina (which also served the people from Cave), San Biagio at Mori, and San Niccolò at Terraio.
Facing the main square stood the church of the whole new community, dedicated to the Virgin Mary, which in 1443 was elevated to the rank of pieve (parish church).

The seventeenth-century plan of the “Enclosure of Terranuova,” the 1734 map by Vittorio Anastagi, and especially the cadastral maps from the Leopoldine period, preserved at the State Archives of Arezzo (now also available online on the “Castore” website of the Region of Tuscany), allow for an accurate and detailed reconstruction of the forma urbis of Terranuova. It displayed the typical features of the Florentine terre murate (“walled towns”): straight crenellated curtain walls pierced by loopholes at regular intervals, interior walkways, corner towers, four fortified gates, and a large central square, the focal point of civic life.

If the private houses had to be erected quickly, the fortification works — to which even the other Valdarno communities under Florentine rule were obliged to contribute — required a much longer time.
When the works were finally completed, after many years, the walls had no fewer than twenty-four towers: four at the corners, four over the gates located at the midpoint of each side, and sixteen smaller turrets spaced evenly along the perimeter.

The rectangular castle measured about 570 by 270 braccia (approximately 330 × 175.5 meters). The defensive walls were surrounded by a moat with a trapezoidal section, 10 braccia wide at the bottom (about 5.8 m), 20 braccia at the top (11.6 m), and 10 braccia deep, set 5 braccia (almost 3 m) from the walls themselves. Between the walls and the moat ran a “buffer strip” known as the carbonaie, which extended along the entire perimeter and allowed defenders, in case of siege, to strike enemies from the flanking turrets if they managed to cross the moat.

The moats were filled with water diverted both from the stream descending northward from the Fratte and through a canal branching from the Berignolo, which, starting from the steccata on the Ciuffenna, carried water to the “Mill of the Countess” (later the municipal mill and then a paper mill) located at the junction of the roads to Pernina and Ganghereto.

All the towers built along the walls — including those over the gates — projected outward to allow for flanking fire, and were open on the inner side to prevent the enemy from using them against the town should they be captured. Their tops were crowned with battlements and equipped with machicolations supported by stone corbels; these, in turn, supported a wooden walkway that ran inside along the entire wall circuit. Some traces of it can still be seen in the surviving sections of the walls, which were likely topped with Guelph-style battlements. At regular intervals, vertical arrow slits opened in both the walls and the towers — still visible in some sections — wider on the inside and designed for crossbow fire.

Each gate, preceded by an outer gatehouse protecting the drawbridge, had a name identifying it, though these names changed over time to reflect circumstances.

The gates — known as those of San Bartolomeo or San Biagio, San Giorgio, San Pietro, and San Niccolò (the latter surmounted by the bell tower, later moved to the tower of San Pietro after its collapse) — were strengthened by defensive structures on the outside and equipped with gatehouses and drawbridges. Their ground positions are today marked by cemented stone strips.
La loro posizione a terra è segnalata oggi da fasce cementate di sassi.

In the past, when the fortifications were still fully functional, Terranuova’s walls had suffered few attacks and were breached only once. In the past, when the fortifications were still fully functional, Terranuova’s walls had suffered few attacks and were breached only once — in April 1432, when the condottiere Bernardino della Carda, formerly in the service of Florence but later hired by the Duke of Milan for a higher price, began raiding the Florentine countryside.

The contemporary chronicler Giovanni Morelli recounts that this man:
andò in Valdambra, et scese a Montevarchi et tolseci Caposelvi, Santo Leolino et altre Fortezze. Dipoi passò Arno, et andò a Terranuova, ci predò, et tolse prigioni et salse ne’ Monti, et prese la Cicogna, et arsela.
Poi andò a far danno da altre parti, finché non fu sconfitto da un altro avventuriero che prese il suo posto al servizio dei Fiorentini: Niccolò da Tolentino.

Over time, with the evolution of military techniques, the walls, towers, and moats gradually lost their original defensive function. The Florentine state began to lease out the carbonaie, which were turned into vegetable gardens, and the towers, whose inner sides were closed off to create small rooms or dovecotes. Of the four original gates, only the jambs of the northern gate remain, rebuilt after its collapse; the others were completely destroyed by German mines during the retreat of 1944.