I was under the impression that skull-unlock traps and the like were independent of the unlocked abilities gained from progressing in the story. However, a friend claims that he bought the dwarven defenders with skulls before he later unlocked them through story.
Can anyone confirm or deny this, as I would like to make sure I plan out my purchases accordingly if this is the case. Again, I was under the impression that the items marked with what appears to be a shield were only unlock-able via story and the skulls were there to be bought.
Any and all opinions or experiences would be greatly appreciated. Thanks much in advance!