The Doctor whistles the first few bars of the Pink Floyd song "Another Brick In The Wall" when he sees Clara scolding the students for playing football near the large chess board. The music video for Pink Floyd's "One Of These Days" takes inspiration from the show.
A banner hangs on the wall saying "A Spirit of Adventure". This is a nod back to the First Doctor story "The Sensorites", in which the Doctor tells then-companions Barbara Wright and Ian Chesterton that their travels in the TARDIS with him and Susan Foreman turned out to be "a great spirit of adventure".
Building on the experience of writing "The Lodger" and "Closing Time", Gareth Robertsand Steven Moffat tried to match the former for threat, feeling there was an excessive amount in Closing Time, which had included back-story for the antagonists at the expense of other themes. For this reason, it was decided that the first draft had the right amount of threat and that it would be unnecessary to add more later