OMD2 - Skull Collector - The Hive Revisited

Orcs Must Die! 2 - The Skull Collector Challenge

So... I failed to score a Perfect Victory on my Live run.
But that map can be 5-skulled without any skull spent. This is how.

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