Illusion of Gaia is an action RPG. That means that while you have the use of items and your power will increase as you defeat enemies, the game's emphasis lies on action and some puzzle solving. Here's a rundown of how the game works.
A: Attack B: Use Item (or: Y: Attack B: Use Item depending on your setup)
There are only a certain amount of enemies in an area. When you defeat an enemy, it stays defeated until you go all the way back to the world map.
Every time you defeat an enemy, it will drop a Dark Gem. Dark Gems are worth a certain amount of Dark Points (look at the top of the screen to see how many Dark Points you have). The larger the Gem, the more Points it's worth. When you get 100 Dark Points, you have gained a life. That means that if your HP drops to 0, you'll restart back at the last door you entered, with half your HP. However, all items will stay gotten, and all enemies will stay dead. This is useful in a dungeon, but if you're in a boss fight, either use herbs or just restart back at a save point.
Very rarely, an enemy will drop a Dark Gem with a heart in the middle which will restore 5 HP. This doesn't happen often, so don't count on it.
At the top left of the screen is Will's HP. Each dot is worth 2 HP; Will can get up to 40 HP. At the top right is the enemy's HP; this only shows up when Will attacks an enemy. In the middle is Will's Dark Points. The number to the left is how many lives Will has, and the two digit number is how many Dark Points so far.
When all the enemies in a room are destroyed, you will get a Jewel. This Jewel can be a sword (raises Strength by 1), a shield (raises Defense by 1) or a heart (raises HP by one and refills all HP). Obviously, defeating every enemy is a necessary task if you want the game to progress at normal challenge levels. Also, when you defeat certain enemies (it's the same enemy every time) a Force will come out and open up a new path.
Press Start to see this screen. The man in the center is Will. The purple faces are enemies. The brown squares are treasure chests. (On the left, you can see how many enemies and chests are left in the room.) The "FORCE" indicates if there's a passage somewhere which will open if you defeat the right enemy (no, it doesn't say which enemy). If you see a blue circle, that's a Dark Space.
Dark Spaces are Will's lifeline in the game. They exist in dungeons and in towns as well. They look like small white balls when Will's far away, but when he comes close they turn into starry portals. Inside is Gaia, the earth spirit, who helps Will by refilling his HP and saving his game. In dungeons, most Dark Spaces will allow Will to change into Freedan, the Dark Knight. In the Pyramid, Will can also change into Shadow, the Ultimate Warrior. See Moves for the likenesses and differences between these three forms. Although the other forms are stronger, Will has special moves which allow for more movement through different paths in dungeons.
Most of the game is linear; Will cannot return to places he's missed. To use the world map, just choose the location you wish to go to. Since each location has a maximum of three possible destinations, you might have to backtrack several times before you reach the place you want to get to. Frequently you have no choice at all in where to go.
Throughout the game you'll be collecting Red Jewels; these items allow you to collect special prizes. See Red Jewels for greater detail on this.
Herbs can refill your HP by 8 points. However, be careful with them: there's a finite amount of herbs in the game and if you use them up early, the later bosses might prove extremely difficult.