- last post: 01.01.0001 12:00 AM PDT
You can unlock multiplayer levels depending upon what difficulty above Easy you complete the game on. I know the when you complete the game on Normal, you unlock the level "Foundation," which many consider to be reminiscent of one of Bungie's older games "Marathon." As far as the other levels go, I'm clueless, sorry.
As for the skulls... here's a link to all the known info so far about the skulls that can be found in the game: Everything We Know About The Skulls! As far as I know, nothing major has been unlocked by collecting all the skulls as of yet. There is a theory that says there may be a sequential order that they must be obtained in to unlock something, but who knows. Just check out the link for any info so far.
-Necro