The titular Black Sage himself is invincible (especially because your character is at level 1 in the game). Ogres and orcs can be encountered in his castle, but you should better try and avoid them, using your stealth, wits and memory (or this guide).
Your goal is to get the highest possible score (experience) and find an exit from the castle. Since no sequel was ever released, "experience points" represent your score.
When you manage to leave the castle, you break the high score if you get at least 3000 points. All the riches you collected are turned into points to that end.