Castell (/kæˈstɛl/ ka-STEL) is a small unincorporated, rural town in Llano County, Texas, United States. It is part of the Texas German belt region, and most residents are still ethnic German-Texan. Its population was 104 at the 2010 census. Located in the heart of the Texas Hill Country, its northern border is formed by the Llano River. It was designated a Recorded Texas Historic Landmark in 1964, marker number 9440.