The plot, as well as the gameplay, revolves around the Book of Prophecy and the touch-sensitive lower screen of the DS. To record a person, monster, plant or weapon in its pages, the player performs a "Code Scan", which is done by hitting the target to be scanned with the Book itself. Each page shows the object's basic information, such as attack strength (for items), health points (for enemies) and personal likes, dislikes, and goals (for people).
In addition, everything that can be scanned has a Code - which is like DNA, but simpler. Its Code is in turn made up of many Codes, which are pieces of different shapes and sizes that fit into the object's grid somewhat like fitting pentominoes into a box. Creating different combinations of Codes, by adding and removing them, changes the object's title and sometimes has a major effect on the object. Changing titles to remove 'defective' Codes, as well as creating 'new' items, is a facet of gameplay used in many sidequests and occasionally within the story.
An example of Code manipulation is as follows: at one point in the adventure the player encounters a writer who has an illness, rendering him very weak. Once he is Code Scanned, the part of his code causing his ill health can be seen and removed, causing the writer's 'miraculous' recovery. The player can then attach the illness to an enemy's code to make every enemy of that particular species, everywhere in the world, weaker by a certain amount. It would also work with a weapon, such as a sword that, when used, poisons any enemy it touches.
Each page in the Book of Prophecy has a value, and once the Book's entire value is reaches a certain level, it will level up. The Mental Map, the portion of an area where Codes are stored, will sometimes expand. In order to raise page values, besides Code manipulation, the player may do things such as examine points of interest on a map, give gifts to a friend, defeat many of a certain enemy, call on that Spirit for advice, or equip that weapon.
Interaction[edit]
Most human non-player characters (NPCs) can be given gifts, and have differing tastes and aspirations. Avalon Code features a total of fourteen 'eligible' NPCs (seven girls to choose from if the player chooses the boy character, seven boys if the player character is female - in other words, five people and two Spirits apiece); two of them are 'secret' and were not listed in the game's instruction booklet. Each character has one or more sidequests that the player can undertake at certain points in the story, in addition, which are mostly fetch quests or reveal a little of the character's backstory. These are accessed by raising that character's affection level, by doing things such as giving them presents.
Dungeons[edit]
Dungeons are fairly linear, and consist of many linked rooms in which the player must complete a specific task to advance (and determine that page's value). There are several types of goals, ranging from lighting torches to defeating enemies, and although there is an onscreen timer for each challenge room, the player may still complete the challenge and advance after running out of time with no penalty other than a low score. Most dungeons, naturally, end with a fight against a boss. Rooms and bosses may be replayed as many times as the player desires in order to achieve a better score and possibly hidden bonuses.
After defeating each monster boss, the player unlocks a new page in a special Book chapter. These pages contain questions about the new world, and the Codes that are inserted as answers will determine different aspects of it. The answers can be filled in differently as many times as the player wishes, yielding different results.