The fortification: castro Tnin (Tnen, Tignino castro) was built on the highest north-eastern plateau of the Spas mountain, probably already in the second half of 9th century. Later it became a mediaeval settlement, the nucleus of the subsequent fortress complex and the Knin town space. Knin was mentioned for the first time in the work by Byzantine emperor and writer Constantine VII Porphyrogenitus: De administrando imperio (On the Administration of the Empire) in 950. Tenen (Knin) was mentioned among nine inhabited towns of the “Christened Croatia”.
In domestic sources, Knin was first mentioned in 10th century in the document by the Croatian king Krešimir I as Tignino. Although some towns in Dalmatia are older than Knin, in cultural and national opinion, Knin is one of the oldest Croatian towns, since its emergence and development are connected to the arrival of the Croats (at the end of 6th and the beginning of 7th century) and Knin is thus rightfully considered to be the cradle of the Croats. Because of that it is no wonder that Croatian rulers chose Knin, first as a temporary, and from Zvonimir (1074-1088) on as the permanent capital of the Croatian state. The Knin diocese was established in 1040. The bishop carried the title “Croatian bishop” and his seat was in Polje in the church of St Mary that was built beside the Knin Fortress. Its jurisdiction extended to the Drava river. After the death of Petar (1097) the last Croatian king who also chose Knin for his capital, many conquerors have invaded Knin, plundered and destroyed it through centuries, till today.
The Knin Fortress is the biggest fortification in Dalmatia. Its original look is not entirely known because of the lack of original documentation. Similarly to the fortifications of the period, it can be assumed that it was surrounded by high walls, and its construction was adapted to the terrain, and in places protected by naturally steep slopes. Its current appearance dates from the beginning of 18th century when it was artificially separated from the Spas mountain that leans on it on the north side. It is divided into upper, middle and lower town, which are connected by drawbridges. The upper town in the north part of the Fortress is the oldest, while the middle and lower towns were built in late Middle Ages. Below the Fortress, the Knin castrum, a civilian settlement with church objects took shape very early – podgrađe Borgo de Tina.
The Fortress contained state rooms, as the “palace” of Croatian rulers, where they issued documents and stayed while they were in Knin. Later on, a smaller fortification – Lab castle (castro Lab, Labwar) was built on the other elevated plateau of the Spas mountain, south from the Tnena fort. It housed the seat of vice-governor. Subsequently, possibly in 15th century during the increasing danger of Turkish attacks, a defence wall was built, on the place where the main entrance into the Fortress is situated even today, connecting these two fortifications, castles, into a unique defence complex. Each part of the town was protected with loop-holes, battle stations and connected with town gates and drawbridges. The oldest known graphical display of Knin (the Knin Fortress) was recorded on the map of north Dalmatia and Lika by a Venetian cartographer Matteo Pagano dating around 1525. However, a more detailed urban layout of Knin fortification displays the drawing by a Venetian military engineer Orazio Antonio Alberghetti, made at the time when the Turks were expelled in 1688. The Knin Fortress is a protected urban unit, entered into the Registry of Cultural Heritage of the Republic of Croatia, as the highest category monument, i.e. the monument of special national interest.