What is "sarkel"

Sarkel (or Sharkil, literally white house in Khazar language) was a large limestone-and-brick fortress built by the Khazars with Byzantine assistance in the 830s. It was named Sarkel, or 'white-house', because of the white limestone bricks used in its construction. Sarkel was located on the right bank of the lower Don River, in present-day Rostov Oblast of Russia.