Castiel is a former municipality in the district of Plessur in the canton of Graubünden in Switzerland . On 1 January 2013 the former municipalities of Castiel, Calfreisen , Langwies , Lüen , Molinis , Peist and St. Peter-Pagig merged into the municipality of Arosa .

Castiel is first mentioned in 1132 as Castellum.

Before the merger, Castiel had a total area of 5.4 km2 (2.1 sq mi). Of this area, 43.1% is used for agricultural purposes, while 49.2% is forested.