StoneEye is a boss in StoneCastle.
This boss is located three screens to the right and one screen up from the first StoneCastle save point.
From the save point, use the red energies to jump up to the next screen, then use the yellow energies to jump past the spikes to the next screen up. Here, jump on the Walking Pillar, past the fireballs, to the yellow energies above (the player needs several jump upgrades, and perhaps all of them, to complete this jump). Go one screen right, one screen down, and one more screen right to the arena.
StoneEye shoots small lasers from him which bounce around the arena, and produces a horizontal laser at the bottom of the arena. Jump to avoid the horizontal lasers and dodge the bouncing ones until three yellow energies appear. Use the yellow energies to jump up to StoneEye and dive-bomb him when able. His attacks will grow faster but remain the same in scope. Repeat attack 4 times to defeat him.