A Zodiac Chamber, introduced in Kid Icarus: Uprising, is a special area hidden in every other chapter, starting with Chapter 2. There are twelve chambers, each of them containing either a specific weapon with specific stats or a power. There are nine weapons and three powers total. To enter one, Pit must locate the Zodiac Platform and stand on it. He will be teleported to the chamber where he will find a specific weapon or power.
Chapter 2: The Sagittarius Bow
Towards the middle of the level, Pit will be walking down a flight of stairs. On the wall, there will be a carpet; if Pit walks through it, he will find the Zodiac Platform.
Chapter 4: The Taurus Arm
In the room that Palutena tells Pit about Clubberskulls, if Pit falls to the bottom floor where there's a Clubberskull, there will be many pits, but falling into the glowing one will reveal the Zodiac Platform.
Chapter 6: The Gemini Orbitars
Chapter 8: The Cancer Claws
If Pit follows the second floor path in the constellation room and falls off the edge, there is a secret doorway behind the largest constellation orb. There will be a jump pad to send him back up.
Chapter 10: The Leo Cannon
Chapter 12: The Virgo Palm
After defeating all of the enemies in the area containing two grenades, two elevators will appear. If Pit takes the elevator behind the place where the grenades were, he will reach the Zodiac Chamber.
Chapter 14: The Libra Sponge
Chapter 16: The Scorpio Staff
Chapter 18: The Aries Armor
Chapter 20: The Capricorn Club
In the jail cell area, there is a cell holding a Skuttler. Pit should open up its cell via a switch nearby and defeat the Skuttler. The back wall of the cell is an illusion that Pit can walk through; the Zodiac Chamber is on the left.
Chapter 23: The Aquarius Blade
When Pit reaches the room that contains many grind rails, Pit can go all the way to the end, and just before reaching Hades' stomach, he can go back on the grind rail he just disembarked. Following that grind rail all the way back to the center area but avoiding the parallel rail to the left will allow him access to the Zodiac Chamber.