From the original perimeter of the Venetian era (1473-1498), the walls with the towers Campana, S. Giorgio, Portello, Spiritata, Marcella, Calcina and Porta Nuova are still preserved, with the addition of the Soccorso tower, built in the Habsburg period (1615); however, the western section of the walls with the Palazzo tower and the Italia gate were demolished.
To defend the eastern border of the Republic of Venice from the Turks, temporary structures were built along the Isonzo river starting from 1473. After the Turkish invasion of 1478, masonry structures were preferred: to the north a bastion was erected, called Emo, to defend the northern gate of the fortress; to the south a wall with towers to protect the Collisello, on the site where the castle stands today.
Porta Nuova was built between 1483 and 1488 and the adjacent old gate was abandoned; in its place, the tower of S. Giorgio, in the basement of which the remains of the old passage are still visible. In 1497, the perimeter of the fortress was closed with the northern section of the missing walls and a new tower, guarding Porta Nuova. Having thus lost its strategic function, in 1508 the fortress of Emo was demolished.
In 1498, the entire outer wall was completed: a fortress with an irregular pentagon shape, accessible by two gates, surrounded by a moat fed by the Isonzo river, protected by thick stone walls with internal walkways and equipped with circular towers.