What prop could you use to describe or interpret the Castle of Elsinore in Hamlet?
This is a tricky question. My answer here deals with props used in the play, not some "creative prop" not seen.
Castle Elsinore is a flurry of activity during the play. It has staged a wedding and a funeral, is visited by ghosts and a group of motley players, is on the verge of war, and is the site of multiple murders. Overall, it is a place of deception.
The most obvious props in the play itself, Yorick's skull and Ophelia's flowers, are associated with death and are staged outside the castle Elsinore, but still on its grounds. The most notable props used inside the castle are swords, daggers, and letters of intrigue. And then there's the "play within a play," called "Mousetrap," and there are masks used, all of which connote deception.
In his brilliant 1996 adaptation, Branagh depicted Elsinore as a hall of mirrors: Claudius, Polonius, and Hamlet pop in and out of scenes from behind revolving two-way mirrored panels. So, I think the mirror is a perfect prop to express metadrama and the play-acting, vanity, and deception of its players.
So, we have props of death and mortality outside the castle; props of violence, intrigue, and deception used inside the castle. You can choose a skull, dried flowers, swords, daggers, letters, masks, a mousetrap, or a mirror. Which best interprets Elsinore? Choose wisely and check for poison.