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!
Well you have your ornate key, so now you need to find where to use it!
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Have you passed through the riddle room, with 4 riddles and alcoves? Solve the riddle by putting the right item in each alcove. The way onwards is through there.
Danel.
A gently fried snail slice is absolutely delicious with a pat of butter...
Did you open the locked door in the dino area? If not,
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To open the door you need to flip a switch in a room that is east and a little north of where the door is; the lever is just a bit further into the dino area from where the door was found.
if you have,
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Make sure you picked up the Scroll of Lightning Bolt that is on the shelf. You don't need the scroll but picking up the item activates a switch.
In the hallways past the Pillared Hallway
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There is a secret door that leads to a 1x1 room with a shelf holding a Book of Infinite Wisdom.
If none of that helps
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You need to have picked up three things in order to move into the final part of the level:
1. Scroll of Lightning bolt
2. Ornate Key
3. Book of Infinite Wisdom
When all three items have been picked up wall panels are lifted to expose the path forward.