The medievalism of "Outlander" is there from the outset. Thoroughly modern Claire Beauchamp Randall and her urbane, ardent husband, Frank, venture to the Scottish Highlands for their second honeymoon. In the pilot episode, Claire's (and Frank's) modernity is emphasized over and over again...and especially when she suggests, with a few glances and a tantalizing lack of undergarments, that her husband go down on her in an abandoned castle (he agrees with enthusiasm). Their egalitarian partnership--sexual and otherwise--and Claire's full autonomy are highlighted against the gloomy medieval backdrop, which itself dates from a time when, supposedly, women were little more than chattel and had no autonomy of their own.
Later, Claire steps through the standing stones at Craigh na Dun and is thrown back in time, to 1743. The first thing that happens to her in 1743 is that she's almost raped by a British soldier and the second thing that happens is that she's rescued by a Scot and then taken prisoner--confirming the notion (for the audience and for Claire) that the past is a dangerous place for women. A mysterious local woman, Geillis Duncan, later warns Claire that the Highlands are no place for a woman alone; this warning stands in stark contrast to Claire's memories of the work she did as a nurse on the front lines in WWII, separated from her husband by their respective war duties. By the end of the episode, she ends up at the same castle--Castle Leoch--where she and Frank had their intimate interlude. Only now, Castle Leoch isn't abandoned. It's the bustling seat of Clan Mackenzie and Claire's new home/prison.
Claire's an Englishwoman in the Highlands, a sassenach (foreigner), and her Scots hosts (or captors) are suspicious of her for that reason. Her asynchrony is her secret, something that only she--and the audience--can know.** Claire's "out-of-time-ness" drives much of the plot: her attempts to escape her Scots guards and evade the sadistic British captain who tried to rape her and return to Frank, and her knowledge of the Jacobite Rising of 1745, which virtually destroyed much of the distinctive Highland culture. The audience learns, through voice over and flashback (or is it flash-forward?), that after the Battle of Culloden in 1745, the Highland clans were broken and their language--Gaelic--forbidden by the British. For Claire and for the audience, the outcome of the Rising is both history and foreknowledge, and Claire's anachronism gives rise to nostalgia for a way of life that will soon be lost.
That way of life, which Claire--and the audience--get to know over several episodes, is distinctly medieval, uninterrupted for centuries. Collum Mackenzie, the laird (or lord) of the clan lives in a castle; his brother, Dougal, is a war-chieftain. Collum has a harper in his service, just like in days of yore. Claire acts as a healer, or Beaton, to the residents of Castle Leoch, and her disgust at the filthy and barbaric implements and remedies that her predecessor used highlights the "medieval" medicine available at the time. She attends the Gathering, when all of the laird's tenants come to the castle to pledge their fealty with solemn vow, and celebrate with games, drinking, and a boar hunt, and she saves a young boy from the clutches of the fanatical and benighted Father Bain by diagnosing him with accidental poisoning rather than demonic possession. Dougal and the men-at-arms (including Jamie) collect the annual rents from the laird's tenants. Despite the fact that specie is widely used, the Mackenzie tenants often pay their rent in livestock, grain, or other goods. The women in one of the settlements chant songs that have come down through generations of women, going back hundreds of years. The laird's man of law, Ned Gowan, admits to Claire that he came to the Highlands from tame, civilized Edinburgh in search of wilderness and adventure. The English soldiers that Claire meets in Brockton call the Scots uncivilized, brutish, and wild--not because they live in the Highlands, but because they live in the past. Their loyalty is to their local laird, not their distant king; they wear kilts instead of trousers; they speak Gaelic instead of English; they live in crofts and villages, rather than towns and cities.
* Talking about the tv show in this post, not the books.
** For now.