The directions The Doctor gives Bill for the wardrobe are the same directions the Ninth Doctor gave Rose in The Unquiet Dead (2005).
Steven Moffat joked in an interview that the intro to EastEnders (1985) was always about a giant snake, a reference towards the view of the River Thames from above, which makes it look like a serpent.
Sarah Dollard wanted to prominently feature a group of street urchins in her narrative, since she felt that historical stories were an effective way to cast light on marginalised communities.
Spider is one of the few child characters to be killed on-screen throughout the history of the program.
Sarah Dollard suggested an episode that explained why the Thames ceased icing over after 1814. In reality, this was principally due to the fact that the British climate was becoming milder as it emerged from the Little Ice Age, which had persisted since the mid-fourteenth century. The 1831 demolition of the old London Bridge was another contributing factor, since its structure had allowed ice to build up and stem the current, which facilitated the freezing process. Likewise, the construction of the Thames Embankment in the middle part of the nineteenth century also prompted the river to run more swiftly. Dollard thought that there might be a more fantastical explanation for the phenomenon, and Steven Moffat suggested the presence of an enormous monster below the waters of Thames. He had always been struck by the serpentine geography of the river, and liked the idea that it conformed to the shape of a giant creature.