How do you recruit 40 royal knights under the influence of a keep

how do you recruit 40 royal knights under the influence of a keep. I build a stable right beside a keep. recruited the 40 knights but did not get the achievement. And YES I was on line at the time. Please reply if you know the EXACT answer

If you’re talking about the mastery, I believe it needs to be the Red Palace specifically. Not just any keep.