The manual states that you need a specific amulet for each castle. In each castle there is a picture which I assumed corresponded to the amulets. However, I always received the amulet for what appeared to be the right castle on the same level. So I suspect there is a mistake with the pictures because in order to successfully complete the game I had to...

1) Kill enemies in a level until amulet is collected (shows up on the bottom right of the screen). There are 5 amulets, one for each level.
2) Don't go into any of the castles before all 5 amulets are collected.
3) After all 5 amulets are collected, go to each castle and kill Red Sabbath. I did them in level order 1-5, just to make sure. The best way to make sure it's working is to check your score. You get a large amount of points for killing each Red Sabbath clone.

That's all there is to it really, but it took me a long time to figure out exactly how to do it properly.

A few other important notes:

-Don't warp to level 3 (middle level) - The jump at the start is very difficult. You can get to level 3 from level 4 by walking left from the castle. You get to the castle by jumping down a hole on level 4 and since you can't jump back up, walking left takes you back to level 3 instead.
-The holes on level 4 and 5 are both safe to jump down. In the room through the hole in level 5 you get to the next screen on the right by ducking and moving right. You have to duck, but not on the way back.
-Pressing F6 to "save" and return to the level select can be quicker than waiting for the bird. (To exit a level otherwise you hold your sword up so that it touches the bird that flies past - don't ask). You can't actually save & load your game though. So don't quit. You need to do the whole thing in one go.
-The manual states that pressing F5 gives you game information. When I tried it it killed me. So don't press it.
-To kill most things, the best way is to just duck and hold your sword out. They'll just keep walking into it.
-There is a hole on the left side of the cave on level 3, right before the platform you need to jump up to. There is also a falling stalactite in that room, so watch out for that. I found that just jumping forwards worked.