Nest of Evil

Geography
The Nest of Evil's environment greatly resembles both a coliseum and a black market fighting arena. This hidden (and optional) area of the game acts much like the coliseum's found in other Castlevania titles in which you head through a series of rooms and do battle with increasingly tougher groups of monsters. And after every third challenge comes an encounter with a semi-boss of sorts whom is a guest appearance from the previous DS Castlevania title: Portrait of Ruin.

This hidden area can be opened only by completing all of Wind's training missions and then selecting the mission "Nest of Evil" to open the path to it.

The area is located in a small hole situated in the entrance room of the castle which beforehand had not existed.