The Zhonecage is the optional dungeon of Tales of Graces f. It consists of 10 floors, which are completely random every time you enter. It is composed of two sections, with a boss at the end of every floor. There are enemies roaming around on each floor, just like in the main game, although they respawn rather quickly inside the dungeon (similar to the Dark Bottle effect). Every big room in the dungeon has a treasure chest in one of the corners, which contains an item usually specific to that floor, and your chances of getting the better quality items will depend on the difficulty setting.
HOW TO UNLOCK
Once you complete the main game, you'll be asked to save at the end of the credits. Load this save back up and you'll be right before the last boss again, only this time you'll see a new skit pop up. This will lead you back to Lhant and you should see a star symbol in front of Lhant Manor. After viewing the cutscene, the Zhonecage should now be available from the world map.
HOW IT WORKS
Once you set foot in the Zhonecage for the first time, you'll be given a brief introduction on how it works. First you interact with the center pedestal, then pick a floor, and then you choose an item from your inventory that is above a certain value to be able to enter that floor. Floor 10 is a special floor, though, which requires that you defeat all 3 cryas dragons first and obtain their respective Valuables first, then you need an item that has a value of around 210,000 Gald.
Once you enter a floor, you have to clear it, meaning you need to get to the end and defeat the boss of that floor. If your entire party dies while on a floor, then you get thrown out to the lobby and will lose your item that you bet. If you succeed, then you will receive your bet item back and be put back on the lobby. Either way, any items you acquire on the floor will remain yours.
You'll also notice on each floor, in the upper right corner, a percentage that will keep increasing as you walk around. This is how much more difficult enemies will be on each floor; it starts out at 125%, but the minimum is 100% and the max is 200%. Every fight you finish will lower the percentage by 10%, and there is another way to lower it.
On each floor, you'll undoubtedly run into some devices, which are both yellow and blue in color. Interacting with these will produce different effects. Yellow ones will reduce the percentage in the corner by 25%. Blue ones have a variety of uses; it will spawn a chest, spawn a monster you will be forced to fight, or it will spawn a Turtlez, which gives you access to a shop. Note that once you exit the Turtlez menu to return to the Zhonecage, you cannot get back in the shop, unless you find another blue device and get another Turtlez to spawn.