Originally built by the Kings of Northumberland to keep out the Vikings. Didn't work, though.
And an utterly useless piece of information from the wikipedia page:
Air quality levels at Bamburgh Castle are excellent due to the absence of industrial sources in the region. Sound levels near the north-south highway passing by Bamburgh Castle are in the range of 59 to 63 dBA in the daytime.
So, now you know.