The Fourteenth and Twenty Fourth Centuries

As far as I can see without having reread it yet, Poul Anderson's historical novel Rogue Sword is set in about 1306. His historical fantasy novel, The Merman's Children, ends in 1322 but its main action had been set decades earlier. His science fiction (sf) novel The High Crusade describes a sequence of events that began in 1345 but its framing passages are set more than a thousand years later. Although these three works do not form a series, I think that an appropriate reading order for them would be:

first, The Merman's Children because it describes the end of Faerie which features in other Anderson novels;
secondly, Rogue Sword because its background is simply historical with no fantasy or sf;
thirdly, The High Crusade not only because its framing passages are set in the future but also because its main action features faster than light (FTL) interstellar travel and alien contact which are major themes of Anderson's futuristic sf.

In The High Crusade, the alien imperialists are like blue Merseians but not quite. Their FTL involves quasi-velocity with gravity control and no FTL equivalent of radio so it sounds like the FTL in Anderson's Technic History. There is a reference to St Dismas, van Rijn's patron saint. The collapse of the interstellar empire is followed by feudalism as happened on Earth after the Roman Empire in The King Of Ys.

The passages set in the future mention an Israeli Empire which does not exist in any of Anderson's other fictitious futures but also mention a sociotechnician which is a kind of scientific advance that he does envisage in other works.