Mission 2 - The Three Gorgons | Piles of Sacrifice and Dungeon Keys - First pass Castlevania: LoS 2 Guide

#1 Keyhole for a Dungeon Key

Red mark indicates the place with a keyhole for a Dungeon Key

The only place you will be able to use Dungeon Key in this mission is just before the boss battle (Gorgon). The entrance is on the crossing where you will find a Soldier Diary - go through the hole in the wall to get into a room where you will find a keyhole.