Sarkel (or Šarkel, literally "white house" in the Khazar language) was a large limestone-and-brick fortress in what is now Rostov Oblast of Russia, on the left bank of the lower Don River.
It was built by the Khazars with Byzantine assistance in the 830s or 840s. It was named Sarkel, or 'white-house', because of the white limestone bricks used in its construction.
In Old Norse sources, such as sagas and runestones, Serkland (also Særkland, Srklant, Sirklant, Serklat, etc.) was the "land of the Serkir", usually identified with the Saracens.
The exact etymology is disputed. Serk- may derive from "Saracen"; from sericum, Latin for "silk", implying a connection with the Silk Road; from the Khazar fortress of Sarkel; or from serkr, shirt or gown, i.e., "land of the gown-wearers". In all cases it refers to a land in the East. Originally, it referred to the land south of the Caspian Sea, but it gradually expanded to cover all Islamic lands, including parts of Africa (and possibly even Muslim Sicily).
View the full Wikipedia page for Serkland