The many good points about Poul Anderson's "Star of the Sea" include:
the character of Veleda;
the colorful setting of contemporary Amsterdam;
the imaginative reconstruction of myths developing from inchoate paganism towards modern Catholicism;
information about the Northern Revolt, a real historical event;
seasonal changes and life in the wilderness;
the story's pivotal position in the Time Patrol series;
its innovative addition to temporal paradox;
evocative passages about the time-stream combined with concrete historical processes.
Addendum: I have said this better before. See here.