Go to stage 4.
On your way through twards the end of the stage you will find 6 of those skulls that spit fire stacked on top of each other. Just past them is a platform that moves like an inverted pengelium going left and right. Just opposite of it is a spiked ball. Get on the platform and jump over the spiked ball landing on the ground against the wall. Break the lower part of the wall to find a bomb. Hit the bomb to ingite it and the wall will come falling down.
Walk forward a bit and you will find 2 spiked platforms that try to crush you. Get on the right platform and as it's going up jump off of it and land on the upper level. Stay on top avoiding the axes and the yellow eye. At the very end of that platform is a candle hit it and inside will be some classic gaming goodness.