This is the forum for helping you out if you're stuck or if you want to discuss the nitty gritty details of the dungeons within Mount Grimrock. Warning: forum contains spoilers!
So on the trial "Time and Tide" at the second part where you
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need to walk along the trapdoors as they open, how do I get past the locked gate? Or how do I get further..I found a secret (I think it was a secret) by hitting a switch on the wall which opens an alternate path, I've tried falling down in the different pits but can't find how to progress.
OK, I am not exactly sure where you are stuck. The plate to throw the rock is at the very end of the trap door path. But I got stuck earlier before getting there. There is a small trap door path before the big one. You have to stand on a plate before and then you hear 3 clicks and a portal appears fora very short time. I tried to throw a rock through the portal, but the rock ended up always to my left. I managed to solve this by shooting a crossbow bolt through theportal. the bolt landed somehow on the switch platform to the left and toggeld the trap doors. So I could walk to the left and replace the bolt with a simple rock. from there I could walk to the large trap door path where you have to throw a rock at the end.
Also if you place a rock on the plate and stand where the portal appears, it will allow you acces to the room on the left. Where you can obtain a book of wisdom (+5 Skill Points).