Sonic the Hedgehog 2 (8-bit)
Rush through a land of sparkling crystal. Break through crystal blocks to find treasure. Use a floating orb to sling you to higher ground.
Here you'll meet with Robotnik himself, in an arena he has prepared especially for you...
— Description from the Instruction manual.
As with all other stages in this game, Crystal Egg Zone consists of two standard Acts followed by a shorter, third Act which contains the Zone's boss.
Crystal Egg is perhaps the least aesthetically offensive of all the final bases Dr. Robotnik's constructed over the years; a zone of glassy, translucent architecture where even the spikes glow in rainbow hues. Blocks embossed with Robotnik's pugnacious visage frequently conceal hidden walkways.
The game's last boss battle takes place inside a lightning chamber, which Dr. Robotnik makes extra-fatal by throwing gyrating, flower-shaped energy bolts into the mix. However, the boss actually isn't that difficult if you stay inside the transport chutes until the chamber clears; at which point you can leap out, attack Dr. Robotnik, and dive back in to safety before his attacks renew themselves.
His final machine destroyed, Dr. Robotnik beats a hasty retreat down an escape chute. Sonic gives chase, but not quickly enough to stop the fat man escaping via teleporter. Just as the hedgehog thinks all is lost, Tails warps in through the teleport terminal, presumably thrown in there by Dr. Robotnik as a means of expediting his retreat.