The Hidden Palace Zone boss is an optional boss in the 2013 mobile re-release of Sonic the Hedgehog 2, replacing Drill Eggman II should the player branch into Hidden Palace Zone rather than finish Mystic Cave Zone Act 2. The player has to hit the Egg Mobile eight times to defeat Dr. Eggman.
Dr. Eggman flies in front of a giant archaic organ. He will hover just out of normal jump height and will follow the player's movements (elevating slightly if they attempt to jump at him). The massive trombone on his Egg Mobile will activate every few seconds. He begins with four descending notes, causing four hazardous spiked balls to fall from the ceiling and land in the water at random points below the arena (they will never land on the far right side of the room, however). The water will then rise over the platform, taking the floating balls with them for a second or two. After being lowered to the original water level, the balls will detonate and emit a vertical splash that must be avoided. After a few more seconds, a long and loud bellow will emit from Eggman's trombone, causing the screen to shake until one larger spike ball descends. This variant homes on the player's character until it also explodes, thrusting a large jet stream into the air. After this, Dr. Eggman will encore indefinitely.
Per usual, eight hits will mute the machine. During the first phase of attack, the player can take advantage of the buoyancy of the water to reach the Egg Mobile, although the small flames at the bottom of the Egg Mobile will damage the player, unlike other bosses from this game. The main method of attack comes in the second phase, where the player can lure the Egg Mobile above the large homing bomb as it erupts. If hit by the plume of water, the Egg Mobile's engines will cut briefly and Eggman will fall, giving the player enough time to land multiple hits before Eggman returns to the air and repeats the process. Super Sonic and Tails can reach the boss easily, negating the need for waiting.
- This boss is referred to as Brass Eggman by fans, in reference to the naming scheme of the game's bosses.
- This appears to be an entirely new boss design by Christian Whitehead, as no concepts for a boss machine were finalized or even intended for Hidden Palace Zone for the original release of Sonic the Hedgehog 2 as mentioned by Craig Stitt, the level designer for Hidden Palace Zone.
- The musical construct in the background appears to be based on the famous organ of Alexandra Palace.
- There is a glitch with inflicting the final hit while the Egg Mobile shakes just after being paralyzed, causing the damage counter to roll over and become indestructible much like the Final Zone from the original Sonic the Hedgehog game.
- This is the only boss in the 2013 re-release of Sonic the Hedgehog 2 not to be battled in Boss Attack Zone.
- Although the boss has no official name, the Sonic Retro community coined the term "Brass Eggman" with a nod of approval.
- Interview with Craig Stitt, the mind behind Sonic 2's Hidden Palac (3 June 2014). Retrieved on 24 September 2017. “Craig Stitt: The [Hidden Palace Zone] background I really can't say what did and did not change, except the addition of the 'Boss/Organ room' at the end. As I said, that was not on the map given me, so I never created any art for it. And 'why and organ' I have no idea. They may have moved a few things around (but I don't know), but as I said, the only thing I KNOW is new (as far as the background) is the Boss Room.”