Goodge Street tube station

Goodge Street was a London Underground station that wasn't on the Circle line, but was within the Circle line's perimeter. Largely because of this fact, it was able to be used a central headquarters for the British Army as they monitored the spread of the Web around the Circle line. However, this meant that once the Web had spread all the way around the Circle anyone at Goodge Street was, as Staff Sergeant Arnold put it, "trapped".

In the Army headquarters off the main platform, the Doctor and Edward Travers renewed their friendship, while he and Alistair Gordon Lethbridge-Stewart got to know each other for the first time. (TV: The Web of Fear)

Behind the scenes
Goodge Street's location relative to the Circle line is made clear during Lethbridge-Stewart's initial briefing in the third episode. There Arnold literally points to a map to get his point across.