During the Wars with Sweden, Biržai Castle was an important point of defence. In 1625, Gustavus Adolphus, king of Sweden, attacked the castle with 8,000 soldiers and it was forced to surrender. The castle was left in ruins and was rebuilt, only to be burnt in 1655. In 1662–1669, it was rebuilt again in Renaissance style. On 9 March 1701, August II the Strong and Peter I of Russia (Peter the Great) signed a pact in the castle to unite their forces against Sweden. However, in 1704 the castle was completely destroyed and was left in ruins until its restoration in the 1990s.