Poul Anderson's Crossovers

We find that I have written many posts about fictional crossovers.

I think that some of the best such crossovers are to be found in Poul Anderson's works:

Holger Danske is introduced in Three Hearts And Three Lions;

Valeria Matuchek is introduced (indeed, is born and becomes a teenager) in Anderson's Operation Otherworld;

Anderson's A Midsummer Tempest introduces an alternative historical Prince Rupert of the Rhine;

in some chapters of A Midsummer Tempest, the alternative Rupert meets Holger Danske, now lost between universes, and Valeria Matuchek, now a graduate student, in the inter-universal inn, the Old Phoenix;

also in the Old Phoenix but in an Anderson short story, "House Rule," Nicholas van Rijn from Anderson's Technic History drinks with Sancho Panza from Cervantes' Don Quixote and with Eric the Red from history;

Sherlock Holmes appears in Anderson's Time Patrol series and, along with several other fictional characters, in the A Midsummer Tempest epilogue.

While in a divergent timeline, Manse Everard of the Time Patrol could have met and crossed over with a character who was a hero of that timeline. Indeed, Lorenzo de Conti, the personal causal nexus, probably meets that criterion.