When the minotaur chases Richter in Stage 2, immediately fall into the first hole to enter the sewers. Complete the level as usual by defeating the Boss. You will then start Stage 3', the alternate version of Stage 3. Every time your path splits in Stage 3', always select the upper route. Collect the Axe subweapon when it becomes available. Use it to cut the vine-covered cage near the skeletal snake that is attached to a wall. Jump on the cage to allow Richter to jump to the upper ledges. Continue along the upper ledges until reach a breakable tombstone that contains the item that unlocks Castlevania: Symphony Of The Night as a playable game at the main menu.