Across The North Sea

Poul Anderson, The Corridors Of Time (London, 1968).

The final causal circle of this time travel novel begins:

Hu reports that a large fleet is approaching Denmark from England across the North Sea;
He and Storm fly to investigate;
in their absence, Lockridge learns that they have kept the prisoner Brann alive for months, tortured by the psychoprobe;
this starts Lockridge on his opposition to the Wardens that culminates with him leading that large fleet to attack Avildaro and its two resident Wardens.

How did Robert Heinlein or Poul Anderson start to think of their circular causality plots? A circle does not start anywhere. Tim Powers and Harry Harrison each wrote one intricate circular causality novel but Heinlein wrote three such works and Anderson wrote - how many?

When Lockridge realizes the full extent of Storm's evil, he deduces that the Koriachs are made immortal. He had begun to wonder about her age in her own age. She had been seen sixty years previously. Few go through the corridors and must spend years or decades in every period. Time travel and immortality are a powerful combination, usually treated separately in sf.