You could recover it from the dungeon of a rival noble's Castle, particularly if there's an alternate entrance like an escape tunnel or sally gate.

It would explain why you need disposable, disavowable, untraceable actors but still retain the traditional adventure theme and avoid mission impossible chicanery which, truth be told, I've rarely seen players do well.

In my experience, sneaking around in castles usually ends like the cell block scene in Star Wars.  "Boring conversation anyway.  Luke!  We're about to have company!"