When Kar says of the Romans "...their work is robbery, slaughter, plunder. They do this work and they call it 'empire'. They make deserts and they call it 'peace'", it is a direct reference to the famous speech made by the (probably fictional) Celtic general Calgacus, from the Roman histories of Tacitus.
Acclaimed Scottish playwright Rona Munro had previously been commissioned by script editor Andrew Cartmel to write the final transmitted serial of the original series, Survival: Part One (1989). She becomes the first person to have been accepted to write for both Classic Who and New Doctor Who (2005).
The Doctor angrily tells Kar that she, "got the deadliest creature on this planet very cross". Although the implication is that he's referring to the Eater of Light, he could just as easily be referring to himself.
The Legio IX Hispana ("9th Spanish Legion") was a Roman army division stationed in Britain in the first two centuries AD. Records of it cease to be available from around AD 120. This is not necessarily surprising, as most Roman paperwork has not survived to the present day. Nevertheless, conspiracy theorists have devised several gruesome stories to explain how the Legion met its end by foul play. The oft-filmed young-adult novel The Eagle of the Ninth by Rosemary Sutcliff is one of the better known uses of the Ninth in popular culture.