The Croats built the fortress near a devastated Roman settlement (Latin: Tignino castro), soon after they settled in the area. Its guardians continuously upgraded the structure to accommodate the defensive needs at that time. It was one of the residences of Croatian monarchs, and possibly became the main residence later, since it was much safer to rule from Knin over the lands of Liburnia and Dalmatia, and to Christianize the pagan Croats in Gacka, Lika and Krbava. This is testified by a handful of churches in the Knin area built in the time of Prince Trpimir.[6]
The fortress contained few ceremonial halls and the palace of the Croatian rulers, in which they issued their documents and lived with their courtiers during their stay in Knin. On the other more raised plateau of the mountain Spas, south from the fortress Tnena, a second, smaller fortification was built; the Citadel Lab (Latin: castro Lab, Labwar) which was the seat of the viceban.
The diocese of Knin was established in 1040 by King Stephen I of Croatia, which spanned the area to the river Drava. The bishop of Knin had also the nominal title as the "Croatian bishop".[2]
The fortress was divided to a small and a big town in the 14th century. The small town was used primarily for defensive reasons, while the big town comprised the flats that were occupied by the town's governors, bishops or župans.[4] Suburbs were located just outside the walls.[4] The oldest section is the upper town on the northern side of the fortress, while the middle and lower towns were built in the Late Middle Ages.
It is possible that, in the 15th century, during the raising danger of the Ottoman advance towards Europe, an additional railing defensive wall was constructed on which the main entrance to the fortress is situated today. Both of these fortifications, citadels, were connected in a unified defensive complex. In May 1522 the Ottomans laid a siege on Knin that ended on 29 May 1522 with an Ottoman victory. At the end of XVII century was conquered by Venitians until 1797 year of the fall of the Republic of Venice.
The oldest known graphical presentation of Knin was recorded on a map of northern Dalmatia and Lika by a Venetian cartographer, Matteo Pagano (1515–1588), in about 1525. However, the more detailed description of the urban appearance of the fortification was handed by another Venetian, the military engineer named Orazio Antonio Alberghetti (1656–1690) in one of his schemes made at the time of the expulsion of the Turks in 1688.
