Door to the Abyss is a music theme composed by Michiru Yamane for Castlevania: Symphony of the Night. It can be heard as the background theme in the Castle Center.
In the Reverse Castle Center, it can be interpreted as prelude for the penultimate boss battle against Shaft (or final boss battle if Dracula's Castle is explored as either Richter Belmont/Symphony of the Night or Maria Renard/Symphony of the Night).