In our world, was a Queen’s adultery considered to be a capital offense by itself? My knowledge about these issues is rather sketchy, but it seems most of the times charges of “imagining the King’s death” or “planning the King’s death” were tacked on in such cases to facilitate capital punishment.

Thanks to @goodqueenaly jogging my memory and giving me the statute on this one!

As with everything medieval, it depends on the time and place, but to take England as our example (because Wars of the Roses), the Treason Act of 1351 codified and clarified the laws on treason (indeed, the long version of the Act’s title is “Declaration what Offences shall be adjudged Treason”), separating out high treason and low treason – the difference being that high treason was death by hanging, drawing, and quartering whereas petty treason was drawing and hanging without the quartering (for men, women got either drawing and burning or just burning). 

Among the various crimes considered high treason was “if a man do violate the King’s companion, or the King’s eldest daughter unmarried, or the wife of the King’s eldest son and heir.” And if the King’s “companion” consented to the violation, she was on the hook (essentially as an accomplice, which is really weird when you think about it for a minute). 

The specific case of Anne Boleyn, which you seem to be referring to, was somewhat complicated by the question of evidence – the Act of 1351 required an “overt act” and Thomas Cromwell didn’t quite have the goods to prove that. So he threw in a bunch of additional charges where the burden of proof was lower, and that’s basically how he got a guilty verdict. And that, plus some of Henry VII”s very personal hangups, is why Anne Boleyn’s bill of indictment was so very long.