The fortress is one of the most valuable and best preserved examples of defense architecture in Dalmatia. It is made of brick because that material was considered to be most resistant tocannonballs, while the foundations are made of stone. Although defense capabilities of the fortress have never been tested in military operations, the structure still proved successful in protecting the city from sea-bound enemy attacks. During the centuries of use, the structure served to various armies and has undergone a number of renovations, some of them necessary only because of the development of arms. It was completely abandoned by the military in 1979 and has been undergoing renovation ever since.
The town of Šibenik has a fortification system consisting of four fortresses:
Only St. Nicholas' Fortress is at sea, at the entrance to the Šibenik port, and the remaining three are on land. St. Nicholas' Fortress was built on the left side at the entrance to St. Anthony Channel (kanal sv. Ante), on the island called Ljuljevac. The island is situated at the entrance to the Šibenik channel across the Jadrija beach lighthouse. St. Nicholas' Fortress got its name from the Benedictine Monastery of St. Nicholas, which was originally on the island, but due to the construction of the fortress, it had to be demolished. At the request of domestic Croat population of Šibenik, the Venetian captain Alojzije de Canal decided to build a fort on the island of Ljuljevac on 30 April 1525. The fortress was designed and built by the famous Venetian architect and builder Hyeronimus di San Michaela. It was built by in the 16th century to prevent Turkish boats from reaching the port. St. Nicholas' Fortress was armed with 32 cannons. However, its imposing appearance and size were a bigger threat to the enemy than cannons ever were.
Stretching over .85 hectares, the fortress is composed of three levels. The ground floor is in stone and the upper level is in brick which was considered more resistent to cannonballs. The northern side is semi-circular while the southern side has eastern and western bastions and is connected to Ljuljevac island. With its unique triangular shape and sturdy construction, this fortress became the strongest fort on the Croatian coast.