When you complete a level, if you earned a high enough score, you will get a smiling Murfy stamp on that level. Here are the minimum scores needed for each level to earn the stamp.

Unlockable	How to unlock
Clearleaf Forest	44501 points.
Hoodlum Headquarters	43701 points.
The Bog of Murk	25901 points.
The Desert of the Knaaren	55501 points.
The Fairy Council	20821 points.
The Land of the Livid Dead	58001 points.
The Longest Shortcut	26889 points.
The Summit Beyond the Clouds	26701 points.
The Tower of the Leptys	48001 points.