Forts thus evolved complex shapes that allowed defensive batteries of cannons to command interlocking fields of fire. Forward batteries commanded slopes which defended walls deeper in the complex from direct fire. Defending cannons were not simply intended to deal with attempts to storm the walls, but to actively challenge attacking cannons and deny them approach close enough to the fort to engage in direct fire against the vulnerable walls.
The key to the fort's defence moved to the outer edge of the ditch surrounding the fort, known as the covered way, or covert way. Defenders could move relatively safely in the cover of the ditch and could engage in active countermeasures to keep control of the glacis, the open slope that lay outside the ditch, by creating defensive earthworks to deny the enemy access to the glacis and thus to firing points that could bear directly on to the walls and by digging counter mines to intercept and disrupt attempts to mine the fort walls.
Compared to medieval fortifications, forts became both lower and larger in area, providing defence in depth, with tiers of defences that an attacker needed to overcome in order to bring cannons to bear on the inner layers of defences.
Firing emplacements for defending cannons were defended from bombardment by external fire, but open towards the inside of the fort, not only to diminish their usefulness to the attacker should they be overcome, but also to allow the large volumes of smoke that the defending cannons would generate to dissipate.
Fortifications of this type continued to be effective while the attackers were armed only with cannons, where the majority of the damage inflicted was caused by momentum from the impact of solid shot. While only low explosives such as black powder were available, explosive shells were largely ineffective against such fortifications. The development of mortars, high explosives, and the consequent large increase in the destructive power of explosive shells. Plunging fire rendered the intricate geometry of such fortifications irrelevant. Warfare was to become more mobile.
The costs involved in creating the fortifications were huge. Amsterdam's 22 bastions cost 11 million florins. Siena bankrupted itself to pay for the adaption of its defences.
New defences were often improvised from earlier defenses. Medieval curtain walls were torn down, and a ditch was dug in front of them. The earth used from the excavation was piled behind the walls to create a solid structure. While purpose-built fortifications would often have a brick fascia because of the material's ability to absorb the shock of artillery fire, many improvised defenses cut costs by leaving this stage out and instead opted for more earth. Improvisation could also consist of lowering medieval round towers and infilling them with earth to strengthen the structures.
It was also often necessary to widen and deepen the ditch outside the walls to create a more effective barrier to frontal assault and mining. Engineers from the 1520s were also building massive, gently sloping banks of earth called glacis in front of ditches so that the walls were almost totally hidden from horizontal artillery fire. The main benefit of the glacis was to deny enemy artillery the ability to fire point blank. The higher the angle of elevation, the lower the stopping power.
The first key instance of trace italienne was at the Papal port of Civitavecchia, where the original walls were lowered and thickened because the stone tended to shatter under bombardment.
The first major battle which truly showed the effectiveness of trace italienne was the defenses of Pisa in 1500 against a combined Florentine and French army. With the original medieval fortifications beginning to crumble to French cannon fire, the Pisans constructed an earthen rampart behind the threatened sector. It was discovered that the sloping earthen rampart could be defended against escalade and was also much more resistant to cannon fire than the curtain wall it had replaced. The second siege was that of Padua in 1509. A monk engineer named Fra Giocondo, trusted with the defenses of the Venetian city, cut down the city's medieval wall and surrounded the city in a broad ditch that could be swept by flanking fire from gun ports set low in projections extending into the ditch. Finding that their cannon fire made little impression on these low ramparts, the French and allied besiegers made several bloody and fruitless assaults and then withdrew.