OMD2 - Skull Collector - Chilled Cavern

Orcs Must Die! 2 - The Skull Collector Challenge

We've come to the end of the challenge !
The last map is Chilled Cavern, from the Are We There Yeti DLC, and it's infamous for its Goblin Sappers !
But the Skull Collector is also infamous for his devious tricks, so shall he prevail without having spent any skull ?

Check out the Challenge's topic on the official forums :
http://forums.robotentertainment.com/showthread.php?41-The-Skull-Collector-Challenge