Klis was not built as a planned fortress and town, but it grew spontaneously without a previously constructed urbanistic concept. Its architectural structures were being upgraded on top of each other as a logical sequence of its historical development. Inheriting location of an earlier prehistoric settlement, Klis was formed in a wide range of time from the late antique to the early 19th century. Over the centuries, it has been through a series of changes gradually evolving from Roman fort, through old Croatian and Croatian-Hungarian medieval town and fortress, to Turkish and later Venetian and Austrian fortress.
At the time of their arrival, Croats certainly found well-preserved Roman forts on the cliff that they could use for defense. There are no fully preserved late-medieval fortifications and buildings in the Fortress as they were often demolished and rebuilt. In 1445, the first known Fortress builders, Korčula masters Marko Pavlović and Ivan Karlović were mentioned. During the time Captain Petar Kružić was defending the Fortress from Ottomans, he was in charge of its construction and maintenance.
Although Klis was exposed to powerful cannon fire in frequent Ottoman sieges that were destroying its fortifications, the Fortress was not significantly damaged by the fall under Ottoman rule. Holding it in their control for more than a century, Ottomans would only occasionally rebuild it after a few uskok attacks. Thus, the Fortress retained its medieval structure until Venetian conquest in 1648, during which it had suffered a great damage from cannon fire.
Period of Venetian government has left a visible trace in shaping a few fortification structures in the first and third part of the Fortress. Therefore, Klis Fortress represents a complex fortification today in which architectural elements of Venetian defense system of the 17th and 18th century prevail (learn more - Fortress architecture).