From Wikipedia:
The game consists of eight levels, each set in the castle of one of the dukes. At the completion of each level, Orin receives a new sword. The player can choose to play each of the first seven castles in any order, though the boss at the end of each is vulnerable to only one sword. It is therefore easier to play levels in a particular order. There are hints about the correct order hidden throughout the game. Only after each has been completed can the House of Ruth be played.
After the House of Ruth has been cleared and the 8 Eyes recovered, the player must return the jewels to the Altar of Peace. At this point the jewels must be placed in a particular order, or the game is lost. Hints about the order of the jewels are also hidden throughout the game.
8 Eyes features a cooperative mode in which one player controls Orin and the other player controls Cutrus. In the single-player mode, the player has limited control of both characters simultaneously, making the game significantly more difficult.
Source: Wikipedia, 8 Eyes , available under the CC-BY-SA License.