The giants are used to play the game and achieve certain goals. There are four: Ocean, Forest, Rock, and Swamp. Each have unique abilities. One of the main systems in the game is biomes, which can be created by certain giants. There are five that you can make: ocean, forest, mountain, desert, and swamp. At the beginning of the game the world is covered in wasteland. Giants are also capable of creating sources for villages, such as plants, animals, and minerals. Different combinations of biomes and giants generate different sources. These sources are turned into resources by the villagers, specifically food, wealth, awe, danger, and technology. Achieving certain levels of these helps finish projects and sustain growth. Players can also make symbioses, which are advantages made by placing certain sources next to each other. These give boosts to the villager's resources.
There are no set rules to the game, but the goal is to sustain as much peaceful life as possible. This becomes easier the more you play, as you must play to unlock more abilities for the giants. This makes sense, because the giants are somewhat godlike, so they have the power to help humanity, but they don't have to do so. The giants are also elemental creatures, so the more of their element that there is, the more power they have. If any giant dies the game is over. This makes sense because each giant is essential and they all work together. For example without the swamp giant you couldn't make herbs, or without the ocean giant you couldn't make animals.
Reus has not yet been used in a classroom setting, but it could be. The simple graphics and controls could make it usable for middle school and higher levels. Teachers could explain real life examples of symbiosis between plants and animals as well as biomes. They could also relate the game to today's uses of resources to sustain life, and whether or not it needs to be altered. The game also teaches the lesson that you can't control everything. After an era ends or the player has helped the villages as much as possible, it's still up to humanity to survive.