Of all the worlds the Kingdom watches, Earth is the only one that is unaware of the fairy realm’s existence. This is deliberate. Earth is considered the most dangerous planet because of humanity’s dodgy influence on it. And everyone in the fairy realm, good or evil, is united on one thing - humans should never know about the existence of their world. The risk of them trying to corrupt the place is far too high. Even the Chief Witch followed this particular rule. This, of course, is why Eileen’s defection came as a shock - of all the places she could choose, why Earth? It’s why her defection is considered an act of treachery - Brankaresh refuses to believe Eileen won’t give the fairy world’s existence away. It guarantees her exile and would be a good form of revenge. Certainly in her shoes it’s what he’d do.
There are areas inside the Palace that are used for the secret testing of spells or potential new ingredients for charms. These areas are sealed magically and physically against intruders. There is a department called the Testers, which explains what they are and do, but nobody knows who they are, again this is deliberate. It has been speculated L’Evallier is one as is Hanastrew but both refuse to comment. The Queen gets to use potential new spells first, followed by Eileen. Eileen has commented about this but didn’t make too much of a fuss about it so the Queen did draw her own conclusions from that. Eileen doesn’t see a problem with hypocrisy - if she can see the usefulness of something, she’ll use it. It is unlikely that the Testers will ever become official, public knowledge. And even Eileen will not quibble with that.