I believe the name "the circus" comes from the fact that the MI6, which is (essentially) the British CIA, supposedly has or had its headquarters at Caimbridge Circus. The link below refers to this.

My understanding is that this derrogative name refers to the political climate of cold war East Germany, due to the tricks and transient nature of the German politicos.

Here is an excerpt from a recent article by Bojan Pancevski in Berlin's "Sunday Telegraph." You can read the whole article by following the link below.

"Hubertus Knabe looks through his office window at the cluster of grey, socio-realist buildings outside. "This whole area did not officially exist 20 years ago; it was simply a white spot on the map," he says.

He is in Hohenschönhausen, a district in East Berlin once reserved for the top echelons of the Ministerium für Staatssicherheit, or Stasi, the notorious secret service of the former German Democratic Republic. The district is widely known as Stasiville.

"The dormitory for their recruits is now a conference hotel," says Mr Knabe, the director of the memorial centre that was once a secret prison complex at the heart of Stasiville. "Those apartment blocks were for lower-ranking officers; the top brass lived a bit to the left, in the two-storey houses."

A lot has changed since the Stasi was abolished in 1990, but not in Stasiville. "They are still here, of course," he says. The guardians of a regime that ceased to exist almost two decades ago are now living in retirement in the place where they once worked."