Question: What does the name of the first castle erected in 13th century, means what in Gaelic?

Related documents: [0]  [DOC] Douglas (occasionally spelled "Douglass") is a common surname of Scottish origin, thought to derive from the Gaelic "dubh glas", meaning "black stream". There are numerous places in Scotland from which the surname is derived. The surname has developed into the given name "Douglas". "Douglas" is a habitational name, which could be derived from any of the many places so-named. While there are numerous places with this name in Scotland, it is thought, in most cases, to refer to Douglas, South Lanarkshire, the location of Douglas Castle, the chief stronghold of the Lords of Douglas. The Scottish Gaelic form of the given name is "Dùbhghlas"; the Irish language form it is "Dúghlas", and "Dubhghlas", which are pronounced ] . According to George Fraser Black, in southern Argyllshire the surname is an Anglicised form of the surnames "MacLucas", "MacLugash" (which are derived from the Gaelic "Mac Lùcais"). [END] 
[1]  [DOC] Douglas Castle was a stronghold of the Douglas family from medieval times to the 20th century. The first castle, erected in the 13th century, was destroyed and replaced several times until the 18th century when a large mansion house was built in its place. This too was demolished in 1938, and today only a single corner tower of the 17th-century castle remains. The castle was the former family seat of the Prime Minister, Sir Alec Douglas-Home. The castle was located around 1 km north-east of the village of Douglas, South Lanarkshire, in south-west Scotland. The remains are protected as a category C listed building. [END]  
Answer: "dubh glas", meaning "black stream"
"dubh glas", meaning "black stream"