Commanding the Volga River in its middle course, the state controlled much of trade between Europe and Asia prior to the Crusades (which made other trade routes practicable). The capital, Bolghar, was a thriving city, rivalling in size and wealth with the greatest centres of the Islamic world. Trade partners of Bolghar included from Vikings, Bjarmland, Yugra and Nenets in the north to Baghdad and Constantinople in the south, from Western Europe to China in the East. Other major cities included Bilär, Suar (Suwar), Qaşan (Kashan) and Cükätaw (Juketau). Modern cities Kazan and Yelabuga were founded as Volga Bulgaria's border fortresses. Some of the Volga Bulgarian cities still have not been found, but they are mentioned in old East Slavic sources. They are: Ashli (Oshel), Tuxçin (Tukhchin), İbrahim (Bryakhimov), Taw İle. Some of them were ruined during and after the Golden Horde invasion.