“If it had been common knowledge among castle builders, then why are there still quite a lot (about 30%) of castles with counter-clockwise staircases?”
Castle guides love the story that clockwise stairs gave right-handed defenders room to swing. No medieval builder ever wrote it down. About 30% of castle staircases turn the other way, including the Tower of London’s, and Clifford’s Tower in York has both. The tour guide’s best line has no source but the last tour guide.