Somerville, Alabama. The Somerville Courthouse was built in 1837 in the Federal style. It served as the county courthouse for Morgan County until the county seat was moved from Somerville to Decatur in 1891. It was listed on the National Register of Historic Places on March 24, 1972. Somerville is a town in Morgan County, Alabama, United States. It is included in the Decatur Metropolitan Area, as well as the Huntsville-Decatur Combined Statistical Area. As of the 2010 census, the population of the town was 724, up from 347.