The game makes use of a mechanic that allows the player to affect the "elemental code" of the world around them by using the Book of Prophecy which is situated on the touch-sensitive lower screen.

[3] Avalon Code received mixed reviews, being praised for its graphics and innovative mechanics, but criticized for its linear story-line and repetitive gameplay.

[8] Each page reveals its subject's essential information such as attack strength for weapons; health points for enemies; local lore for maps; and personal tastes and goals for people.

[9] Creating different combinations of Codes by adding and removing them to attain specific quantities or ratios changes the object's title and, often, effects.

Changing titles to remove 'defective' Codes as well as creating 'new' items (usually by following certain obtainable recipes) is a facet of gameplay used in many side-quests and occasionally within the story.

In order to raise page values, the player can, for example, manipulate Codes, examine 'Exploration Points' on a map, present another cast member with things he or she enjoys collecting, defeat a high number of a certain enemy, call on a Spirit for aid, or equip a weapon.

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).

In the course of the campaign, Yumil may date characters labelled as Heroines in the Book of Prophecy (kind but terminally ill orphan Fana, determined heiress Princess Dorothea, guarded psychic Nanaida, precocious elf Sylphy), while Tia may date one of the Heroes (wary street kid Rex, vigilant aspiring hero Duran, emotionless swordsman Anwar, devoted commander General Heath).

Six additional characters become available as love interests after the completion of the main quest: two Book Spirits apiece, along with a Hero and Heroine not listed in the game's instruction booklet.

[25] IGN called the game's core component, the Book of Prophecy, "a compelling mechanic you won't find anywhere else".

[20] Nintendo Life praised the game for its "stunning" visuals, but criticized the Book of Prophecy for being "ridiculously annoying [to browse] through to find the data you need".

[3] GameSpot referred to the Book as "the game's most unique and fascinating feature", but said that overall "Avalon Code is a thoroughly mundane adventure despite some interesting, but poorly implemented, concepts".