Orcs Must Die! 2 - The Skull Collector Challenge
Balls and Chains have thick skulls, that's for sure !
Trying to collect them anyway through achieving a Manaless Perfect Victory on Breakneck Triad, a map from the Family Ties DLC, with no skull spent (ie, no upgrades).
Check out the Challenge's topic :
http://forums.robotentertainment.com/showthread.php?41-The-Skull-Collector-Challenge
