Konami Game Boy Collection Volume 1 - Platform: Gameboy Color

Bonus rooms (Castlevania Adventure) 
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Level 1
Locate the third set of rolling eyes. Climb the rope into the ceiling. 

Level 2
Locate the bridge. Continue moving downward until reaching a location 
where rolling eyes are falling to the right with stairs leading in the 
same direction. Destroy one of the eyes so it explodes on the second 
step from the bottom. Fall down the small hole that is created by the 
explosion. Climb down the invisible rope into the bonus room. 

Level 3
Just after escaping from the third set of spikes, a room where your 
character is in a narrow passage on the left with a solid wall to the 
right will appear. Climb the rope almost to the top, then jump off to 
the right and through the wall. 

Level 4
Locate the room that seems empty except for a large figure that throws 
boomerangs. Climb the rope on the right about half way and jump off it, 
back to the middle. Your character should land on an invisible floor. 
Follow this floor to the center of the screen and climb an invisible 
rope through the ceiling. 
Bonus life (Castlevania Adventure):
Leave the first ten candles in level one. Whip the eleventh candle to 
collect an extra life. 

Unlimited lives (Castlevania Adventure)
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Locate the secret room in level 1 that contains a rope. Climb the rope 
to the top of the screen and through the ceiling. Collect the extra life, 
then find the extra life that appears later in the level. After both are 
collected, intentionally die and repeat this procedure. 

Level select (Contra)
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Press Up(2), Down(2), Left, Right, Left, Right, B, A, B, A, Start at 
the title screen.