.. Dropping the mandrake cage

I think the chain is the key. It is attached to the wall inside the jail.

Maybe you could detach it from the wall. You just can't reach it.

With something long. You will find that item at some point.

When you enter Santa's castle. ( There is a long pole in the workshop. )

Use the pole to detach the chain.

Next, you should open the cage. ( Use the guard's keys to open it. )