I don’t think it really matters where in the castle it is, except that If it’s in an upper floor, they could potentially push it out a window or something to get it out.
At the dinner with Strahd, they told Strahd this about van Richten (he didn’t know what the beacon does), and convinced him to have his minions deliver the skull for them.
She thought that van Richten would come to find her if they lit the beacon, because he’s the one who told her about it. I left it where it is RAW, but had Ezmerelda know all about it.