Grimrock

I am outside a gate, after going back up to Level 7 and finding the hidden door that leads to this area on Level 8. One problem: I can't get through the gate to where the spider is lurking with the lightning balls. I have gone over the rest of this level (I think), but still no way to get into this ...

Forget it. Just figured you have to fight your way through it

Not sure what to do here. When going to Level 4, you hit a pad and the gate closes behind you. Then the shrooms come after you. Been trying to kill them for hours. Did I miss something here?