Enclosed by a double curtain wall— a virtually unique solution in the Marches, where settlements were normally surrounded by simple walls—Mondolfo’s walled town retains the original layout designed by Siena-born architect Francesco di Giorgio Martini, who envisioned a mighty warlike structure capable of being defended by a small group of men. No wonder Francesco Guicciardini referred to it in his History of Italy (written between 1537 and 1540) as ‘the most powerful and finest walled town of the Vicariate, prominently situated on a hill and surrounded by noteworthy moats and boundary walls, for which the site functions as a terreplein’.
The preexisting fortification (castrum), dating from the Byzantine period, is reflected in the oval-shaped inner ring of walls currently measuring 420 metres and enclosing a 1 hectare area. The town has quite a regular urban layout with two streets meeting perpendicularly at the centre of the enclosed area (cardo maximus and decumanus maximus, both perfectly oriented). Some of the present-day street names clearly derive from the ancient layout, such as Vicolo del Decumano. The original settlement only consisted of four blocks probably measuring 120 x 90 metres overall (nowadays they measure 120 x 100 metres, as the western section has a slight ‘bulge’ creating a fifth block). In building on the existing fortifications, Martini designed the outer ring of walls—still largely accessible via the patrol paths—with a distinctive and symbolic almond shape, with sloped ramparts and a cordon lining the top to equip them for flanking fire.
The entirely accessible Strata Magna or Via Grande street (now Via XX Settembre – Via Garibaldi) once joined Porta S. Maria gateway to the town square, while a slightly sloping path—the present-day Corso della Libertà—favoured the circulation of carts carrying supplies and munitions as it joined Porta Fano gateway to the central square with the Town Hall and Civic Tower. The complex fortification system—with the outer ring replicating the inner ring (not enlarged to the north and west), shaped like an irregular foursided almond measuring roughly 714 linear metres and enclosing a 3-hectare surface—reveals how the walled town was adapted to the evolving warfare requirements emerging in the last decade of the 15th century.
This can be seen in certain elements of the so-called ‘transitional military architecture’, of which Francesco di Giorgio Martini was among the leading exponents. Whereas Porta Nuova, one of the three gateways to the walled town, already featured in 16th-century records, Bastione S. Anna bastion only began to be built in 1531, following the devastating effects of the siege by Lorenzino de’ Medici in 1517. Its design was inspired by the ideas of Francesco di Giorgio Martini, who had envisioned Mondolfo as a perfect and formidable war machine equipped with the most advanced defensive systems of the emerging Renaissance era.