Unfortunately, this game is a bit more complicated than simply recovering the 8 jewels from each of the evil dukes. You have a choice of what order you want to play the first 7 levels (You must leave the House of Ruth for last) and the order you choose is very important.
When you complete a castle, your sword is replaced with a "more powerful" one from the duke you defeated. It isn't really stronger against the normal enemies you encounter, but it does twice as much damage to one other duke. Which one? That's the trick...if you pick the wrong one, you will have a really hard time winning a battle against him. Even if you do somehow survive, you will then get another new sword that will be effective against some other duke, meaning that you will no longer have an easy battle with the boss that you should have fought before.
To make this long story shorter, you should play the levels in the order suggested in this guide. If you do that, then you will always fight the dukes using the best possible sword.
Before you start, here are some tips that are applicable at all levels.
When you first enter a castle, your life and weapon power meters are only 6 blocks long. If you want to have any chance to complete the level, you will want to find as many hidden jars as possible to increase the length of these meters to as many as 10 blocks.
Most of the enemies you encounter will attack by running towards you until they are close enough to hit you with their weapon. Then, after swinging it a few times, they move again, then swing some more, and so on. You can take advantage of this pattern to keep them from hitting you as much. When the enemy stops moving, immediately back away so you're just out of his range. Then, wait while he takes a couple of steps towards you before hitting him with your sword. Once you connect, quickly back out of his range again. The later you hit him, the more time you will have to back away. Why? Because scoring a hit makes him flash. He can't hurt you until he stops flashing!