In CODEX Naturalis, you must continue the work of the illuminating monk Tybor Kwelein, assembling the pages of a manuscript that lists the living species in primary forests. Can you put the pages together in the best order possible? And are you prepared to sacrifice a species to develop your manuscript? In the game, each player starts with a single card on the table, a card that shows some combination of the four possible resources in the middle of the card, in the corners of the card, or both. Players also have two resource cards and one prestige card in hand, while two of each type of card are visible on the table. On a turn, you place a card from your hand on top of one or more exploration zones that are on cards you already have in play. An "exploration zone" is a fenced-off corner of the card; your starting card has four such zones, one in each corner, while resource and prestige cards have only three. Resource cards have no cost to be played, and they often depict resource symbols in their exploration zones. Prestige cards deliver points when played, but they often have a resource requirement, e.g., three leaves or two wheat/one water/one stone, and you must have those resources visible in your manuscript at the time you play the prestige card. You score points from this card immediately, with some cards having a fixed value and others a variable one depending on how many of a certain symbol are showing or how many exploration zones you covered this turn. If you wish, you can play a card from your hand face down; such a card has four exploration zones, but provides no resources or points. After you play, draw a face-up card or the top card of either deck to refill your hand.

When a player reaches 20 points, you complete the round, then each player takes one additional turn. Players then score points based on how well they matched two public objective cards and one secret objective card, after which the player with the most points wins.