Top-backroute: Skulls has a backroute by stretching 2 builders over the top, without help from any other skills. This backroute has become possible since 0.7 physics.
But even in 0.6 physics, similar things (that don't solve the map) were possible here:
Miner cancels the builder.
You have a spare miner and a spare cuber in the 0.9 top-backroute.
(http://www.lixgame.com/etc/skulls-0.6-top-backroute.png)
Skulls should be designed much clearer such that these things clearly fail (instead of barely fail or even barely work).
-- Simon