The first earth and timber fort was raised in Ostróda (Osterode) by the Teutonic knights in the mid-13th century. It stood in the fork between the Drwęca river branches, replacing an earlier Prussian stronghold. Construction of a brick castle built on a stone foundation was started by the Ostróda Commander (Hauskomtur) around 1349. The old earth and timber fortress together with the unfinished brick castle were hit by a fire during a Lithuanian raid commanded by duke Kiejstut in 1381. Soon afterwards a new brick castle was built.
The convent castle was constructed on a square plan, 44.7 x 45.2 m externally. Originally it had four wings with an archway in the west wing. Recently, the castle has been restored in the shape it had after the fire in 1788. Three wings (south, wet and north) have been preserved, although they lack the topmost storey, which was used for defense and storage. Some researchers claim that the castle had a tower in the south-east corner. To the north, there was a sanitary tower, most probably connected with the north wing of the castle by a wooden walk. During the archaeological excavations at the castle, the bases of pillars were unearthed, which may have supported the wooden gallery leading to the sanitary tower. The south wing contained a chapel, the commander's quarters and a refectory. The remaining wings most probably held the chapter room, infirmary and dormitories for brother knights and guests. The communication within the castle was through wooden walks parallel to the internal sides of the walls. Rooms on the ground floor in the north and south wings have preserved original cross and ribbed vaults on arches supported by granite pillars (partly reconstructed), while the cellars underneath are topped with cross vaults on diagonal arches. After the battle of Tannenberg, for two months Ostróda castle remained in the hands of Duke of Masovia, Janusz Mazowicki, who received the castle from the Polish King Władysław Jagiełło. In September 1410 the castle returned to the Teutonic Knights. In 1629 the Swedish King, Gustav Adolph resided at Ostróda castle. For six years, from 1633 to 1639, the castle was ruled by the Silesian Duke of Legnica and Brzeg, John Christian.
In the great fire of Ostróda in 1788, the flames reached the castle, where large quantities of gunpowder were stored in the east wing at that time. Soon a gigantic explosion damaged the east wing and the castle roofs. Although the castle was rebuilt, it did not regain its previous shape - the east wing was not reconstructed and the whole castle was one storey lower. In the 19th century the walls were plastered and many extensions were added to the castle building. From 21st February to 1st April 1807 the castle hosted the French emperor, Napoleon Bonaparte. He arrived here to rest after the exhausting battle at Prussian Eylau, which was victorious for the French party but caused much bloodshed. A rather unexpected outcome of that brief stay at Ostróda castle was a number of arts objects documenting the event. The Ostróda Museum exhibits a replica of the painting by the French artist Marie Nicolas Ponce-Camus 'Napoleon promises his grace to Ostróda burgers. March 1807'; the original canvas hangs in the historic gallery at Versailles. Another exhibit at Ostróda Castle which commemorates Napoleon I is a silver medal 'Napoleon a Osterode', made by the French graver Bertrand Andrieu. In the 19th century the castle was a seat for the local administration office and court of justice. There were also several apartments for officials. In 1945 the castle was burnt by the Soviet Army and remained in the state of preserved ruin for nearly thirty years. The reconstruction of the castle was commenced in 1974 and continued until the 1990s. At present, the castle houses the Museum of Ostróda, the Centre of Culture, a gallery and a library.References:
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.