Grimrock

In the beggining of the level 3, there's a pit. I jump into it and there's the first problem: on youtube you can see that the enemies here are slimes. Mine are not: they are undead warriors. Furthermore, there is no skull behind the portal that can be deactivated with a lever. Where I can find it?

you must have dropped the warriors down the pit from the other roomand they squished the slimes. The skull is there, around the corner out of sight.