The Schlei is a narrow inlet

Inlet

An inlet is a narrow body of water between islands or leading inland from a larger body of water, often leading to an enclosed body of water, such as a sound, bay, lagoon or marsh. In sea coasts an inlet usually refers to the actual connection between a bay and the ocean and is often called an...

 of the Baltic Sea

Baltic Sea

The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

 in Schleswig-Holstein

Schleswig-Holstein

Schleswig-Holstein is the northernmost of the sixteen states of Germany, comprising most of the historical duchy of Holstein and the southern part of the former Duchy of Schleswig...

 in northern Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

. It stretches for approximately 20 miles from the Baltic near Kappeln

Kappeln

Kappeln is a town in the district of Schleswig-Flensburg, in Schleswig-Holstein, Germany. It is situated on the north bank of the Schlei, approx. northeast of Schleswig, and southeast of Flensburg....

 and Arnis to the city of Schleswig. Along the Schlei are many small bays and swamps. The important Viking

Viking

The term Viking is customarily used to refer to the Norse explorers, warriors, merchants, and pirates who raided, traded, explored and settled in wide areas of Europe, Asia and the North Atlantic islands from the late 8th to the mid-11th century.These Norsemen used their famed longships to...

 settlement of Hedeby

Hedeby

Hedeby |heath]]land, and býr = yard, thus "heath yard"), mentioned by Alfred the Great as aet Haethe , in German Haddeby and Haithabu, a modern spelling of the runic Heiðabý was an important trading settlement in the Danish-northern German borderland during the Viking Age...

 was located at the head of the firth

Firth

Firth is the word in the Lowland Scots language and in English used to denote various coastal waters in Scotland and England. In mainland Scotland it is used to describe a large sea bay, or even a strait. In the Northern Isles it more usually refers to a smaller inlet...

 (fjord

Fjord

Geologically, a fjord is a long, narrow inlet with steep sides or cliffs, created in a valley carved by glacial activity.-Formation:A fjord is formed when a glacier cuts a U-shaped valley by abrasion of the surrounding bedrock. Glacial melting is accompanied by rebound of Earth's crust as the ice...

), but was later abandoned in favor of the city of Schleswig. A museum has been built on the site, telling the story of the abandoned town.