Kaunas Castle is a medieval castle in Kaunas, the second-largest city in Lithuania. Archeological evidence suggests that it was built during the mid-14th century in the Gothic style. Its site is strategic—a rise on the banks of the Nemunas River near its confluence with the Neris River. About one-third of the castle stood at the beginning of the 21st century.

The precise construction date of the first Kaunas Castle is unknown. Archaeological data suggests that a stone castle was built on the site during the middle of the 14th century.^[1] Situated on an elevated bank near the river junction about 100 kilometers (62 mi) from the capital city of Vilnius, it served as a strategic outpost. It guarded nearby cities as well as trade routes.

A written account states that in 1361, the Grand Master of the Teutonic Knights Winrich von Kniprode issued an order to gather information about the castle, specifically the thickness of its walls, as preparation for an assault on the castle. In March–April 1362, the castle was besieged by the Teutonic Order. During this attack, the Knights constructed a siege tower and erected wall-penetration machinery; primitive firearms might have been used since gunpowder technology was emerging in Europe. At that time, the castle walls were over 11 meters (36 feet) high when its firing galleries were factored in. According to Wigand of Marburg, the castle's garrison consisted of about 400 Lithuanian soldiers, commanded by Vaidotas, son of Duke Kęstutis. After three weeks, the Knights breached the castle's walls and soon took the castle. On Easter Sunday in 1362, the Knights conducted a Mass at the castle to commemorate their victory.

Apparently, of the castle's defense force of 400, only 36 survived. Questions remain about the castle's defenders' lack of support from outside during the siege. In any event, Kęstutis soon regained and rebuilt Kaunas Castle, but it remained a point of contention between Lithuanians and Teutonic Knights for many years. In 1384, Kaunas Castle was re-captured by the Teutonic Knights. At this time, Grand Master Konrad Zöllner von Rotenstein began the reconstruction of Kaunas Castle and renamed it Marienwerder. The Knights' presence in Kaunas threatened the entire defensive system of castles along the Nemunas. Confronting this situation, the Lithuanians attacked the castle later the same year.

The Lithuanians likely mustered an army near Vilnius as a strategic maneuver since Lithuanians could use the downstream flow of the Neris River to transport artillery and military provisions from Vilnius; the Knights likely used overland or upstream transport. During the 1384 assault, the Lithuanians deployed cannons and trebuchets; the besieged Teutonic Knights had also installed cannons in the castle, which destroyed the Lithuanians' trebuchet. Nevertheless, the Lithuanians took the castle.

After 1398, the Teutonic Knights could no longer reconquer the castle. After the Battle of Grunwald, Kaunas Castle lost its strategic military importance and was used as a residence. The castle served administrative purposes after the death of Vytautas the Great. Sigismund Augustus gave this castle to his wife Barbara Radziwill in 1549. During the 16th century, the castle was strengthened and adapted to new defensive purposes by constructing an artillery bastion near the round tower. The diameter of the bastion was about 40 meters (130 feet) and the height of the bastion's walls was about 12 meters (39 feet); the wall worked in conjunction with a defensive trench. A firing gallery linked with the tower was installed at the bottom of the bastion.

In 1601, Kaunas Castle housed courts and an archive. At some time in 1611, the Neris River flooded part of the castle.

Due to its convenient location, it was used by the Swedish military during its war with the Polish–Lithuanian Commonwealth, after which its military functions ceased. In the mid-17th century, large portions of the castle were again flooded. The castle was used as a prison in the 18th century; later, the Russian administration granted permission for houses to be built on the castle's territory, which resulted in significant damage to the castle.

Kaunas' city and castle were mentioned in the famous Lithuanian^[2][3] and Polish poet Adam Mickiewicz poem Konrad Wallenrod (1828), set in the 14th-century Grand Duchy of Lithuania.

For many years afterward, Kaunas castle stood abandoned. In the 1960s, the round tower was opened as a museum, but due to its structural deterioration, the museum was transferred elsewhere.

Protection of the castle began in 1930; nearby houses were demolished, and archeologists examined the territory. Further efforts to preserve the castle were made in the 1950s. The round tower was repaired; the firing bastion was later excavated from beneath several overlying strata. The excavated bastion was in excellent condition. As part of its protection, temporary roofing was placed there and on the remaining towers and walls. The remaining portions of the round tower were not reconstructed to their original height, nor were the castle walls; the bigger parts are only the remaining foundations of the walls.

Archeological excavations continued at Kaunas Castle, interrupted by periods of inactivity. The evidence gathered from these archeological works suggests that the castle's configuration, excluding the bastion, has remained in the form it took during its reconstruction in 1376.

Major reconstruction work started in 2010 and ended in 2011.^[1] In 2011, a branch of the Kaunas City Museum was established in Kaunas Castle. The castle is open to tourism and hosts occasional festivals. A new sculpture "Freedom Warrior [uk]" (represents the Vytis) was erected near Kaunas castle on 14 July 2018. The "Freedom Warrior" is almost seven meters high and was cast in Ukraine. A bronze horseman with a horse was placed at the castle of Kaunas on a granite pedestal. The authors of the sculpture are Lithuanian Arūnas Sakalauskas [uk; lt] and Ukrainians Boris Krylov and Olesius Sidoruk [uk].^[4] In 2019 Kaunas City museum with Kaunas Castle became a new Member of Association of Castles and Museums around the Baltic Sea.

• List of castles in Lithuania
• Gothic architecture in Lithuania
• Kaunas Fortress