While the Peninsula Castle was still being built, reinforced, and expanded, construction of the Island Castle began in the second half of the 14th century on one of the larger islands in Lake Galvė. In its first construction phase, the Island Castle was of a transitional type, from an enclosure castle to a convent-type castle, comprising U-shaped ducal palace buildings and a forecourt. It is believed that the construction of the Island Castle was initiated by Kęstutis and completed in the first half of the 15th century by his son Vytautas. The castle was built as a fortress for defense and was never taken by enemies.
In the second construction phase, the castle was redesigned: a six-story donjon tower measuring 9,2 × 9,6 meters (33 meters high) was planned between the palace wings, and a defensive wall with buttresses was built along the edge of the island. The castle’s architecture is Gothic, though some elements reflect Romanesque features. All rooms had vaulted ceilings, and the window frames were decorated with profiled bricks. On the second floor of the palace’s right wing, there was a representative hall with stained glass windows. The first written mention of the architecture of the Island Castle, like that of the Peninsula Castle, was made by Flemish traveler Ghillebert de Lannoy, who noted that “the second castle stands in the middle of another lake, a cannon shot away from the old one. It is entirely new, built of brick in the French style.” In the final construction phase, a moat was created to separate the ducal palace from the forecourt, which was surrounded by a defensive wall with towers and casemates. The southern and triangular casemates housed kitchens, the lower floors of the western casemates were used for storage, and servants lived on the second floor. A prison was located in the southwest tower of the castle. The forecourt acquired an irregular trapezoidal shape.
On the eve of the Battle of Grunwald (1410), the expanded castle featured innovative defenses, the corner towers of the forecourt were adapted for flanking fire and housed 15 cannons. After the defeat of the Teutonic Order in the battle of Grunwald, the Island Castle became the residence of the Grand Duke of Lithuania. In the early 15th century, Trakai was a thriving city, visited by merchants, noble guests, and foreign envoys, who were received in the Island Palace’s representative hall. In 1413, Emperor Sigismund's envoy Benedict Makra was received in the castle, he had been appointed arbitrator in a border dispute over Samogitia. Between 1413 and 1430, the King of Poland and Vytautas’ cousin Jogaila visited the castle thirteen times.
In the 16th century, after losing its military and residential functions, the Island Castle declined in importance. The Grand Dukes visited it increasingly less often. The Lithuanian Metrica, which had been stored in the castle, was moved to Vilnius in 1511. The castle was repurposed as a prison for noble individuals.
Following the devastating Russian invasion of 1655–1661, the town of Trakai was looted and burned, and the Island Castle was damaged and never rebuilt. The town did not recover and became a provincial settlement.
In the 19th century, the rise of Romanticism sparked interest in Lithuania’s past, particularly in the castles of Trakai. Artists painted the ruins of the Island Castle and its remaining wall fresco fragments, while architects and engineers developed restoration projects. Engineer B. Malevskis prepared a project for the partial restoration and conservation of the southeastern tower of the forecourt. Although the work completed was limited, it was significant for the history of preserving the Island Castle. From 1929 to 1941, restoration work was led by Polish engineer and architect J. Borowski. During this period, the base of the masonry was reinforced, some corner buttresses were restored, debris was cleared from the facade, the donjon was stabilized, and reconstruction of the representative hall began.
After World War II, Lithuanian restorers continued the conservation and restoration of the Trakai Castles. In 1953, restoration of the Island Castle began. In 1962, the central palace was rebuilt based on a project by architect B. Krūminis, and in 1987, under the leadership of architect Stasys Mikulionis, the forecourt restoration was completed. The reconstruction of the forecourt’s corner towers and western casemates transformed the castle’s entire spatial composition. The Island Castle, a symbol of Lithuania and Trakai, regained its 15th-century appearance. Since Lithuania regained independence, the castle has once again hosted honored guests and witnessed the signing of agreements.
In 1962, the Island Castle was transferred to the Trakai History Museum. Exhibitions were installed, and the castle became a venue for displays, concerts, festivals, and celebrations. Each year, thousands of visitors come to explore the castle.
