The Northern Gate is of course the Norse route from Iceland to Greenland to Vinland (North America). Greenland has sometimes been identified as Ultima Thule.

“By the late middle ages and early modern era, the Greco-Roman Thule was often identified with the real Iceland or Greenland. Sometimes Ultima Thule was a Latin name for Greenland, when Thule was used for Iceland.”

So this comic takes place sometime after the voyages of Leif Erikson.

Æthelred the Unready would be King of England.
His Wife was Ælfgifu (Elf Gift) of York

This would be the start of the High Medieval Period.

Mayan civilization (where the City of Blood is located) is in its Post Classic Period, a time of decline (which in this story is due to the gods abandoning the world after the Elvan pranks at the Bodensee during the start of the story)

Since Ultima Thule (the trans-Neptunian object) has been in the news recently, here is a picture of it.

NASA Ultima Thule Rule 34