The Château d'Aguilar is one of the so-called Cathar castles. The design of the castle witnesses the practical military thinking of the 12th century. The castle consists of an inner keep built in the 12th century, surrounded by an outer pentagonal fortification from the 13th century. This fortification is oriented such that its point guards the side most favourable to attackers. The keep and the inner hexagonal fortification is flanked at each corner with semi-circular guard towers, each equipped with archery outlooks. The strategic location of the castle on a hill overhanging the plain of Tuchan allows supervision of the Corbières. Despite this, the castle is easily accessible from the plains because of its relatively low elevation of 321 metres.
There is a small underground chapel of Saint-Anne below the keep.
The earliest building at this location belonged to the count of Fonnollède since 1021. In the 13th century, the keep that had replaced earlier buildings was bequeathed by the viscounts of Carcassonne to their vassal, the Ternes.
In 1210, it was invaded and occupied by Simon de Montfort, whose soldiers took and held the owner Raymond de Termes in a dark dungeon in the Carcassonne. Militarily, the castle lay dormant for the next 30 years, until Raymond's son Oliver de Termes took back the castle in the brief revolt against the crusaders. Aguilar became the refuge of many faydits, Cathar knights and lords without strongholds. In 1246, a royal garrison was installed to supervise the Aragon frontier.
Olivier, however, eventually made an alliance with king Louis IX, who purchased the castle from him in 1260. Despite the heavy fortifications, the castle would be continually under siege by opposers to the French or Spanish rulers until the 16th century.
When the border was pushed back to the south of Roussillon by the treaty of the Pyrenées, the castle gradually lost its strategic importance, and was eventually abandoned in 1569. Today it is in decrepit condition. Since 1949, it has been listed as a monument historique by the French Ministry of Culture.
From its origin as a small stronghold built by the ancient Illyrian tribe Dalmatae, becoming a royal castle that was the seat of many Croatian kings, to its final development as a large fortress during the Ottoman wars in Europe, Klis Fortress has guarded the frontier, being lost and re-conquered several times. Due to its location on a pass that separates the mountains Mosor and Kozjak, the fortress served as a major source of defense in Dalmatia, especially against the Ottoman advance, and has been a key crossroad between the Mediterranean belt and the Balkan rear.
Since Duke Mislav of the Duchy of Croatia made Klis Fortress the seat of his throne in the middle of the 9th century, the fortress served as the seat of many Croatia"s rulers. The reign of his successor, Duke Trpimir I, the founder of the Croatian royal House of Trpimirović, is significant for spreading Christianity in the Duchy of Croatia. He largely expanded the Klis Fortress, and in Rižinice, in the valley under the fortress, he built a church and the first Benedictine monastery in Croatia. During the reign of the first Croatian king, Tomislav, Klis and Biograd na Moru were his chief residences.
In March 1242 at Klis Fortress, Tatars who were a constituent segment of the Mongol army under the leadership of Kadan suffered a major defeat while in pursuit of the Hungarian army led by King Béla IV. After their defeat by Croatian forces, the Mongols retreated, and Béla IV rewarded many Croatian towns and nobles with 'substantial riches'. During the Late Middle Ages, the fortress was governed by Croatian nobility, amongst whom Paul I Šubić of Bribir was the most significant. During his reign, the House of Šubić controlled most of modern-day Croatia and Bosnia. Excluding the brief possession by the forces of Bosnian King, Tvrtko I, the fortress remained in Hungaro-Croatian hands for the next several hundred years, until the 16th century.
Klis Fortress is probably best known for its defense against the Ottoman invasion of Europe in the early 16th century. Croatian captain Petar Kružić led the defense of the fortress against a Turkish invasion and siege that lasted for more than two and a half decades. During this defense, as Kružić and his soldiers fought without allies against the Turks, the military faction of Uskoks was formed, which later became famous as an elite Croatian militant sect. Ultimately, the defenders were defeated and the fortress was occupied by the Ottomans in 1537. After more than a century under Ottoman rule, in 1669, Klis Fortress was besieged and seized by the Republic of Venice, thus moving the border between Christian and Muslim Europe further east and helping to contribute to the decline of the Ottoman Empire. The Venetians restored and enlarged the fortress, but it was taken by the Austrians after Napoleon extinguished the republic itself in 1797. Today, Klis Fortress contains a museum where visitors to this historic military structure can see an array of arms, armor, and traditional uniforms.