Lake Širvėna is the oldest artificial body of water in Lithuania, created in 1575 by damming two rivers; it guarded the approaches to the fortress and the town. The lake's origin is linked to the construction of the Biržai defensive fortress. Spreading over a marshy area, the nearly 400 ha pond reliably protected the castle and town from the north. In 1928, when a flax factory was set up in the buildings of Astravas Manor on the other side of the lake, a 525 m long and 2.45 m wide footbridge was built across Lake Širvėna, linking Astravas with the town. Today Lake Širvėna covers about 325 ha, and the 525 m bridge across it is the longest pedestrian bridge over a lake in Lithuania. The bridge is popular with anglers, newlyweds, locals and visitors who want to admire the town and the calm of the lake and walk across to Astravas Manor.