What does the name of the first castle erected in 13th century, means what in Gaelic?
Douglas (surname): Douglas (occasionally spelled "Douglass") is a common surname of Scottish origin, thought to derive from the Gaelic "dubh glas", meaning "black stream". Douglas Castle:  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.
"dubh glas", meaning "black stream"