Under the palace.
If you keep it a secret, a whole dynasties may spend the time defending the secret without knowing it even exists. (muahahha)
Alternatively, if you are lord of the palace, you have it readily available so you can use your tomes of eldritch knowledge against your enemies (muhahahaaha)
Unless your hidden magical stash is the dead drop stash of sorts, then you want to have it far away where you go to pick up things where everything goes to shit.
Otherwise, it is better to have your magical artifact available.
If I were an evil wizard, I would certainly want to have my spellbook handy.