M. C. Escher

Maurits C. Escher was an artist famous for creating confusing engravings. (AUDIO: The Forever Trap)

Graham Stevens had an array of Escher prints in his office. (PROSE: Cat's Cradle: Witch Mark)

The Seventh Doctor told Ace that she did for the English language what Escher did for spatial geometry. (PROSE: Lucifer Rising)

Laura Corbett said that the Land of Fiction was "very M. C. Escher". (AUDIO: The Crooked Man)

The Tenth Doctor compared the Edifice to "the steps that go up and up and join up". (AUDIO: The Forever Trap)

The Eleventh Doctor, while trapped in the Bank of England, jokingly remarked that it was designed by Escher. (GAME: The Eternity Clock)

Behind the scenes
The television story Castrovalva is likely named after the M. C. Escher's print. The titual city of Castrovalva featured confusing spatial geometry seen in M. C. Escher's work.