 Complete the following phrase:  In Sassanid times, the region became a major cultural center due to the wealth derived from the Northern Silk Road. Sassanid rule was interrupted by the Hephthalite invasion at the end of the 5th century and didnt return to the Sassanids until 565. Many Persian nobles and landlords escaped to this region after the Muslim conquest of Persia. It was also ruled by Göktürks until the 