Hamelin no Violin Hiki

While on their quest to stop the Demon King, the violinist Hamel, the pianist Raiel, the magess Flute and the female warrior Sizer stop in a small kingdom and destroy a rampaging monster. While giving the four a heroes' welcome, the king and queen explain that their daughter has been kidnapped by a monster living in a nearby castle. They agree to go and rescue the hostage princess and defeat the monster. However, the enemy's castle is heavily guarded...
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(Source: Anime News Network)