The siege of Bilär was a battle for the capital city of Volga Bulgaria, between the Volga Bulgars and the Mongols. It took place in the autumn of 1236 and lasted 45 days. It ended with the destruction of Bilär and the massacre of its population, estimated at several dozen thousand.

After the Battle of Samara Bend, the Bolghars renovated Bilär's fortifications. The third 11-kilometre-long wall of stone and wood encircled the city. However, after the Mongols besieged the city, it withstood the siege only for 45 days.

By the materials uncovered by archaeological excavations, the city was burnt after falling, and the unburied remains of its population were found all over Bilär. The excavations prove the Kazan Tatar legends and the Russian chronicles, which wrote that the Mongols:

…взяша славный Великий город Болгарьский и избиша оружием от старца и до унаго и до сущаго младенца, и взяша товара множества, а город пожгоша огнем, и всю землю их плениша.

…took the glorious Great city of Bolghar and massacred everybody, from monks to babies, and took many goods and set fire to the city and captivated their land.

Then, the Mongols destroyed many Bulgarian cities, but the country's north remained intact, so many survivors resettled to the North and the West from the Bulgarian mainland. The country was incorporated into the Ulus Jochi, but the resistance lasted forty years.

Some years after the fall of Bilär, Bolghars tried to revive the Great City, but this attempt had no result.

• (in Russian) История Татарстана, Казань, "ТаРИХ", 2001.
• (in Russian) История Татарской АССР, Казань, Татарское книжное издательство, 1980
• "Bilär/Биләр". Tatar Encyclopaedia (in Tatar). Kazan: The Republic of Tatarstan Academy of Sciences. Institution of the Tatar Encyclopaedia. 2002.