Klis is the key to the Dalmatian coast. It rises on a sharp cliff that closes a narrow passage (Klis door) between rocks of Kozjak and mountain peaks of Mosor. Precisely on that narrowest part, below the northern cliffs of the fortress, remains of the route of the Roman and medieval road are preserved, as well as "Vetmin most" (Bridge of Vetma) over which once passed "Sinjska rera" - railway connection between Split and Sinj (1903 to 1962). Fragments of prehistoric and Greek ceramics confirm the existence of an Illyrian settlement, which guarded the main passage from the interior to the coast. Last archaeological excavations have revealed the remains of an ancient cistern, which archaeologically places the ancient fortress for the first time at the very top of Klis rock or around the today's church of St Vitus. This fortress played a major role during the Byzantine-Gothic Wars in the 6th century, but a century later it failed to defend Salona from the Avar-Slavic campaigns. Croatian princes and kings also recognized the importance of this place, and built there one of their seats. During the research of St Vitus' church, decorated pieces were found of the altar screen belonging to the tiny Old Croatian church with the name of the previously unknown Croatian Queen from the 10th century, Queen Domaslava. During the Middle Ages the fortress frequently changed hands: the Templars, noble families Šubići and Nelipići, Croatian-Hungarian kings and governors. In 1537, after years of resistance by Petar Kružić, the fortress was taken by the Turks who stayed there, with a short interruption, until 1648.
Most of the buildings existing today on the fortress date to the period of Venetian reconstruction during the 17th and the 18th century. The fortress has an area of about 8840 m² and it spreads with almost its entire surface of the elongated cliff in the direction northeast - southwest. It has a length of about 305 m, and its maximum width is 53 m. Considering the distribution and organization of space, it is a complex type of fortification, consisting of three mutually separate forts, each of which has its own entrance and can function independently.
The main, western entrance to the fortress dates from the 17th century, while the entrance to the medieval fortress was under the so-called Oprah Tower, on the site of today's second entrance. The most important is the third defensive enclosure. It is the highest and the best fortified part of the fortress, with the entrance fortified by towers, with ruins of the drawbridge and towers on the upper parts of the fort. This was the last point of defense, but also the center of the fortress. Storage of weapons and equipment, apartments for officers and soldiers (some barracks were also located within the second defense wall), providur's apartment or the prince's house, and the aforementioned church of St Vitus, once a medieval church on whose ruins a Turkish mosque was built, were all located here.
In addition to its indelible importance for national and European history, Klis Fortress represents one of the best preserved late-medieval and baroque fortification entities, which kept its military and strategic importance almost to the Second World War.