in Bloodstained: Ritual of the Night will lead you back intothe castle from the Tower of Twin Dragons area. There's a very useful ability to collect here, as well as an NPC to talk to. Livre Ex Machina connects to Tower of Twin Dragons, Dian Cecht Cathedral, and Hall of Termination....
you'll need to be in the upper area of the Hall of Termination. You will find a destructible ceiling in the very top room. Once inside make sure you grab the chest containing the Celeste Key. There will be a second destructible wall on the left side that will lead you to...