Grimrock

What are you supposed to do with the spider that steps on the pressure plates shooting lightning bolts at you?
there is an iron gate in that room and also one of the holes can be closed, but i have no idea what to do, i dodged the bolts for like 20 minutes and nothing happened

i just found a button in that room with the spider and the lightning bolts but it seems to do nothing

Youve got to defeat him with ranged weaponry ... try throwing a horde of rocks =p ... if you dont have ANY ranged weaponry, i dunno what you can do, except backtrack and collect a load.

but nothing happens if you kill it... and there is obviously a corridor behind the holes that you can access somehow