This is a selection for my book group, so I'm not going to go into very much detail here.
What a fascinating mish-mash of science fiction and fantasy! Our hero Simon Tregarth finds himself on the run from gangsters in present-day America (well, 1960s) and travels through a fairy circle to a world that somehow meets his fondest desires. The world he ends up on has elements of highest fantasy--with witches guiding their soldiers into war after preparing their way with their supernatural powers--and speculative fiction or planetary romance. The valiant Estcarpians (Estcarps?) with whom Tregarth settles find themselves at battle with what at first appear to be an army of the undead, a fitting foe for the witch women. But it turns out it is not magic that has removed the minds of their foes but instead some horrible science this is beyond anything that Tregarth has ever experienced before.
Unfortunately, Andre Norton only hints at the possible technology until the last few dozen pages, so she is unable to exploit the dichotomy very far, and I can only hope that it gets further explored in the many many novels that follow Witch World. But in those final battle sequences, the culture of Estcarp, apparently somewhere near our Middle Ages, finds itself at battle with an enemy using aircraft and submarines. The nearest thing I can recall to the battle is William Forstchen's Lost Regiment books, featuring a Civil War regiment versus nomadic tribes. Unlike Forstchen, Norton appears to play favorites in the battle, or perhaps she is just advising on the benefits of going into battle fully prepared.