Roots: Day credits Robert Marshall's Storm From the East as a key account of the Mongol invasion. There are references to Augustine, Coleridge and Sherlock Holmes.
Continuity: The Bunker Soldier was created to fight in a war on a planet located light years from Earth that ended centuries before 1240 AD. The war was fought between different ethnic groups within a species and the Bunker Soldiers were created to differentiate between the ethnic groups of its victims by sampling their genetic material via the bone needles extruding from its hands and head. The Soldiers, described by the Doctor as soldier, assassin, and spy rolled into one would be launched in their capsules towards the impregnable cities, or Bunkers, of the enemy group. On reaching its target, the Soldier would kill any members of the enemy ethnic group it encountered, but its ultimate aim is to locate the leaders of the Bunker communities and use its psychic abilities to induce madness in them, thus creating widespread carnage. One such soldier could infiltrate and destroy an entire Bunker. One such Bunker Soldier was launched in its capsule, but was shot out of the planet's gravity and eventually reached Earth. It crash-landed in Russia six generations earlier and its capsule was placed in the catacombs beneath the Cathedral of St. Sophia in Kiev. It is a tall skeletal humanoid with a skull-like face and leech-like mouth filled with bone needles, which it can use to kill. It also has vicious talons on its stunted fingers and toes. Its skin is described as constantly moving and re-forming and it can change its shape to impersonate humans. It has grey-blue blood. It can communicate with its capsule telepathically, and the capsules control device automatically tries to establish a psychic communication protocol with Steven when he is carrying it. The Soldiers are designed to disintegrate on completion of their mission; the Soldier in Kiev self-destructs when the Doctor reprograms its capsules control device so that it tells the Soldier that its mission success index has reached 100%. The race the Soldier was created by had three digits on each hand and blue-brown skin.
The Doctor rides a horse competently, albeit with some discomfort (see Marco Polo). He has studied Earth history by reading in addition to actually visiting different eras.
Steven is familiar with Sherlock Holmes, so Conan-Doyle's stories are still being read by his time (unless he has been spending time in the TARDIS library). He isn't sure whether he believes in Heaven [and it is implied, God] having seen little evidence for it.
Dodo is unfamiliar with Coleridge. She unwittingly gives a crazed Dmitri the idea of catapulting plague-ridden corpses over the walls of Kiev at the attacking Mongols, thus possibly (although not definitely) instigating the Black Death.
The TARDIS air-supply is constantly recycled, purified and modified (see Planet of the Daleks). The Doctor says that his ship is ninety per cent indestructible (see The Shadows of Avalon). A subsection of one the panels on the control console rotates, revealing an array of sockets and connectors capable of connecting to a variety of alien devices.
Links: When discussing changing history with Steven, the Doctor mentions the Monk (The Time Meddler, The Daleks' Master Plan). Steven recalls their argument in the TARDIS after leaving Paris (The Massacre). The Doctor promises to tell Dodo of his encounter with Marco Polo and Kublai Khan (Marco Polo).
Location: Kiev, over several months during 1240 AD.
Future History: By Steven's time, illness and premature death have been eradicated. [or at least the kind of illness and premature death he sees in Kiev.]
The Bottom Line: An astonishingly good pseudo-historical novel from Day, whose previous The Menagerie left much to be desired. The characterisation of the regulars is excellent, as is that of the supporting cast, not one of who is lacking in credible motivation. Steven in particular benefits here, due largely to the sequences told in the first person from his point of view. The setting is well conveyed, and as with Asylum it is only the needless inclusion of the alien presence that detracts from the whole.