Biržai Castle is a castle in Biržai, Lithuania. Construction of the earth bastion-type castle started in 1586 by the order of Krzysztof Mikołaj "the Lightning" Radziwiłł. In 1575, preparing for this construction, a dam was built on the Agluona and Apaščia rivers at their confluence, and the artificial Lake Širvėna, covering about 40 km², was created.