Roman Britain is one of the historical periods that is fairly central to the British imagination, so one might have expected Doctor Who to have visited there quite a bit. But, as I argue in a piece in the latest Terrible Zodin, Doctor Who, at least in its early years, went out of its way to avoid the obvious historical periods. It is true Malcolm Hulke, co-writer of the successful Target Luna/Pathfinders sf serials for ITV, submitted an outline for a serial set in Roman Britain (referred to as 'Britain 408 AD', a purely descriptive title), which at one point was planned to be the sixth serial in the first season. But even that avoided the standard periods for Roman Britain on screen (which I discuss here), instead setting itself in 408 CE (or 400 according to a summary David Whitaker wrote in September 1963), at the end of Roman Britain. When the final years of Roman Britain do appear on screen, they are generally connected to the reign of King Arthur (who will then be treated as a historical figure), but David Whitaker's summary of Hulke's story makes no mention of Arthur. In any case, Whitaker changed his mind about this serial, feeling it was overcomplicated, with an ending too like that of 'An Unearthly Child' (the full story), and instead commissioned Hulke to write up another submission, 'Hidden Planet', about a duplicate Earth the other side of the Sun. This was in turn abandoned in 1964. Hulke resubmitted 'Britain 408 AD' in 1965, for the second season, but it was rejected by new script editor Dennis Spooner, because by this point the programme had already made 'The Romans', and Spooner didn't want to repeat himself. (Hulke would have to wait until the Patrick Troughton era and 'The Faceless Ones' for an onscreen credit.)
Over much of the show's existence, Roman Britain was something only vaguely alluded to. The Doctor and his companions are thought to be from Britannia in 'The Romans' (1964), because Vicki and Barbara are overheard talking about London/Londinium. Similarly, in 'The Fires of Pompeii' (2008) Donna is thought to be Celtic/Welsh when she tries to speak Latin. You could argue that the Romans who menace the Doctor at the end of Episode Two and beginning of Episode Three of 'The War Games' (1969) are in Britain - it's not stated, and the zones in 'The War Games' are from conflicts from all over the planet, but the sequence was filmed in Sussex (the Doctor says the Roman time zone was 2,000 years ago, which would rule out Britain, but it's not clear whether he is being precise, and whether he means that in relation to 1917, the time zone from where they crossed over). In 'The Stones of Blood' (1978) the Doctor makes reference to having read Caesar and Tacitus on the Druids when discussing Druidism with sect leader De Vries. And in Battlefield (1989), the Doctor gets mixed up with post-Roman Arthurian legends, though suggesting that they had their origins in the eighth, rather than fifth or sixth centurires.
But the first actual definite television visit of the Doctor to Roman Britain is in 'The Pandorica Opens' (2010), where the Doctor meets River Song at Stonehenge in 102 CE. Even this doesn't engage closely with the typical periods of Roman Britain on screen, and has to be treated as one of the outliers, along with things like Chelmsford 123. The typical periods have been engaged with by non-television Who; so audio adventure 'Wrath of the Iceni' takes place at the time of the Boudican revolt (and is a fully-fledged 'pure' historical, with no aliens interfering with history), and another audio, 'Living History', takes place during Julius Caesar's invasion (a rare non-comedic treatment of Caesar's landing, perhaps due to the presence of a Dalek).
All of which is an excessively-long preamble to talking about 'The Eaters of Light', the point at which Doctor Who most closely engages with the tropes of Roman Britain. It engages with the supposed 'disappearance' of the Ninth Legion. As you may know, there are essentially two theories about what happened to the Ninth. Theodor Mommsen suggested that it was destroyed in some battle in the north of Britain. The discovery in the twentieth century of evidence suggesting that part of the Ninth was present in Nijmegen until c. 120 CE called Mommsen's theory into question (and it's hard to believe that, had he known of the Nijmegen material, he would have ever come up with the idea in the first place), so while the disappearance of the Ninth in Britain continued to fuel popular culture in the form of Rosemary Sutcliff's 1954 novel The Eagle of the Ninth, or the 2010 movie Centurion, the scholarly consensus rejected the notion. However, in recent years, scholars such as Miles Russell, Neil Faulkner, Nick Hodgson and Simon Elliott have come out in favour of the Mommsen idea.
Interestingly, Rona Monro has the Doctor (Peter Capaldi) and his companion Bill (Pearl Mackie) take different sides of the argument - Bill, who did a school project on the Ninth, believes that they left, whilst the Doctor says they were wiped out. The Doctor is, of course, proved right, though, being a modern Doctor Who story, they were destroyed by an alien menace.
Ninth Legion stories are often connected with Hadrian's Wall, but not here. The location of the action is moved to somewhere near Aberdeen (far further north than most people think the Ninth might have penetrated). But that location, as Juliette Harrisson has observed, allows the episode to be a love letter to Scotland, from where star, showrunner and writer all hail (though Monro is the native of the Granite City, Capaldi and Steven Moffat coming from Glasgow and Paisley respectively). A series of standard Scottish jokes about the weather are trotted out ('It's Scotland. It's supposed to be damp.'); but these also fit a standard trope of Roman Britain, where it rains all the time. Another standard trope of Roman Britain is its remoteness from the rest of the empire; here this can be linked to the perceived remoteness of Scotland from the rest of Britain.
Other standard trope are to be seen. The Picts around Aberdeen (Picts are actually anachronistic for the second century) all have their faces painted. There is a Boudica equivalent in teenager Kar. The use of child protagonists is also something I have realised is to be seen a lot in Roman Britain tales.
And then there's folk horror (here, I am in considerably sympathy with Louis Bayman and K.J. Donnelly, who argue in the introduction to their edited collection on Folk Horror On Film that the defining characteristic of folk horror is that the horror arises from the people, customs and practices of the folk themselves, rather than anything outside that). Who has, of course, engaged with folk horror on many occasions, starting with 'The Dæmons' in 1971, continuing through 'The Stones of Blood' and 'The Awakening' (1984), through to more recent fare such as 'Human Nature'/'Family of Blood' (2007, directed by the same man who does the job here). So, much of 'Eaters of Light' takes place in dark woods, and though the threat is external, it is bound up in the Picitish traditions. Nobody mentions Druids, but there is a stone circle and a cairn, the Devil's Cairn, in fact (a name with echoes of Devil's End from 'The Dæmons').
There is also a great deal to enjoy and appreciate. The Roman survivors are ethnically mixed, as they might well have been. They have names like 'Cornelius' and 'Lucius'. Which are the sort of names screenwriters give randomly to Romans, but they are actually the sorts of names, citizens' names, that members of a legion would have. That Bill is considered a bit weird not for being a lesbian, but for not being bisexual is a nice moment, though I'm not sure how rooted it is in Roman social mores. But I did enjoy the callback to the speech of Calgacus from Tacitus' Agricola: 'They make deserts and they call it peace.'
The Doctor claims to have lived in Roman Britain, to have 'governed, farmed, juggled', all presumably in adventures not seen on screen (but perhaps even now being written for Big Finish). He also claims to have been a Vestal Virgin, second class. Given that this is before it had been established that the Doctor had been a woman in the past, one wonders precisely what is meant by that.
In general, there is a lot of food for thought in this episode. The main issue with it is that, like much of modern Who, the story doesn't really have time to breathe properly. It could have done with a second episode.