The castle stands on a small natural hill called Collisello, south of the historic centre near the Isonzo. It is enclosed by a wall that encloses buildings from different periods: the Palace of the Captain ('500), the Venetian Arsenal ('400), the Barracks called "La Longa" ('600), the Austrian Barracks ('800), the Guardhouse ('700) and the Church of S. Giuseppe ('800).
Originally, in 1478, during the Venetian period, there was only a deposit for ammunition and a small fort for its defence. Subsequently, a house for the captain of the fortress was built in 1481.
After Gradisca passed under the Habsburg Empire, in the first decades of the 16th century the construction of the castle began with the Palace of the Captain, originally a two-storey building strengthened at the corners by four square towers. The walls were completed in 1572 and differ from the external one of the city because it is equipped with corner towers rather than circular ones, a sign of the evolution of military techniques.
In the 19th century, the castle was used as a court and prison and underwent some changes, including the addition of new buildings, which were then demolished during a restoration in 1979.