There is a long-running dispute as to whether Kazan was founded by the Volga Bulgars in the early Middle Ages or by the Tatars of the Golden Horde in the mid-fifteenth century, as written records before the latter period are sparse. If there were a Bulgar City on the site, estimates of its foundation range from the early 11th century to the late 13th century (see Iske Qazan). It was a block-post on the border between Volga Bulgaria and Finnic tribes (Mari, Udmurt). Another vexed question is where the citadel was built originally. Archaeological explorations have produced evidence of an urban settlement in three parts of the modern city: in the Kremlin, in Bişbalta in the place of modern Zilantaw monastery and near the Qaban lake. The oldest was the Kremlin which could be dated back to the 11th century. In the 11th and 12th centuries, Kazan could have shielded a Volga trade route from Scandinavia to Iran. It was a trade center, and possibly a major city for Bulgar settlers in the Kazan region, although their capital was at the city of Bolğar further South. In the 13th century, re-settlers came to Kazan from Bolğar and Bilär area, which had been ruined by the Mongols. Kazan became a center of a duchy, which was a dependency of the Golden Horde. In 1430s Hordian Tatars (such as Ghiasetdin) usurped power in the duchy, which was ruled by Bolghar dynasty before.