
The Vikings
As the Vikings attack, in the Japanese version:
The Planets by Gustav Holst, and the particular piece is "Mars, the Bringer of War"
In the French/English versions, it's In the Hall of the Mountain King by Edvard_Grieg (I recall this being asked a few times here in the past years... I find this piece bewitching and apparently I'm not alone!


I had taken a few notes of the music from various episodes, but I don't have time to browse through them at the moment. Oh, just one to make sure I won't forget it:
Robot Land
Each time Devil King appears, Night on a Bald Mountain from Modest Mussorgsky is played.