This is the first time since the classic series that The Doctor opens the TARDIS doors using the control panel. The sound effect used is also the same as in the classic series
When the TARDIS goes into "siege mode", it becomes a miniature version of the Pandorica.
It was only after writing several drafts that Jamie Mathieson was told the episode would need to lock the Doctor away in a single location, as Peter Capaldi's scenes for the episode needed to be filmed quickly to bide by the production schedule.
Jamie Mathieson was a lifelong fan of the show and had previously had discussions about writing for seasons one and five without success. His work caught the attention of Sue Vertue, who convinced her husband Steven Moffat to give him another chance. Aware that his earlier suggestions had dwelt too much on high concepts at the expense of offering engaging adventures for a general audience, Mathieson developed four concepts to present to Moffat. He discussed them with Toby Hadoke, whom Mathieson knew from his days as a stand-up comedian, and was reassured that the series had not previously explored similar ground.
Jamie Mathieson was inspired by the 1533 painting The Ambassadors by Hans Holbein the Younger. Although ostensibly a portrait of diplomats Jean de Dinteville and Georges de Selve, the artwork was best-known for the presence of a large skull, rendered in anamorphic perspective such that it could be clearly interpreted only by an observer standing at an oblique angle to the canvas.