The Ahom Kingdom was founded in the 13th century by migrants from northeastern Burma who conquered the old kingdom of Kamarupa. The kingdom was able to withstand invasions from the Bengal sultanate and even the Mughal empire, although the king was forced to pay some tribute, and Aurangzeb even briefly issued coins in Assam. Eventually, the kingdom was defeated by invading Burmese armies in 1821–22. At that point, the British stepped in and made Assam a British protectorate in 1825. The earliest coins from the Ahom kingdom date from the 15th century. Originally, the coins were round, but in the 16th century, the shape of the coins was made octagonal, supposedly because the Yogini Tantra mentions that Kamarupa is eight-sided. The octagonal shape and the long series make these coins highly popular with collectors.