Grimrock

Fill the altar of Mukka and take the food in the alcove by the starving prisoners. Go to the area that opened by the stairs and break the cave in. Drop down to level 3 and find the wall button, then teleport up again. Throw something over the pit and go to the other side. There you'll find a blue gem.

Next, feed the prisoners (anything will do) and you are teleported to the caged ogre. Pull levers to open doors, and if you're clever about it you can cage the ogre so that you won't have to fight him. There is an alcove by the teleporter out, put something in it, find the secret wall that opens and you'll find a red gem.

(Now you'll have enough gems for the "feed me and me brothers" part when you go down to level 3. Two gems are found on level 1. You can feed the daemon heads and take back the gems later on when you've gotten the goodies from the secret room by the stairs up to level 2.)

You'll fidn the Thief's moon down there, bring it to the altar of Lunare and cross the pits to find a red gem and a green gem.

Cross the teleporter mace and break the cave in. Push the button to open the two gratings. Remove what's in the wall alcoves and put something back in. (The lesson about 'to give is to get' once again.) A secret door opens and there you'll find a lever that turns off several teleporters, among them the ones around the blue gem.

After the Pjofa altar you need to "give to get" once again. A secret wall opens and you'll find a red gem. (This is also where the "Pjofa's key" is found.)

After the altar of Otta, in the fire ball hell. You'll find a green gem on the floor by a secret button.
Push the button so that at least one fire ball reaches all across the room and the door at the south east of the room opens, there you'll find a red gem.

There should be a note somewhere about filling the pits in the pit area. The easiest thing to do is to put something you don't need in all pits. That will close the pit to the south west and there you'll find a red gem.

Dropping down to level 3 from the altar of Isin you'll find 4 gems in different hidden rooms. There are a lot of secret buttons to find to find them all.

Arriving at level 2 again you go straight forward and find a secret button in the corridor. That opens a secret wall later on. Fill the altar of Rumu and to the south west you'll find three alcoves, fill them and another secret wall opens. Teleport to the catacombs. Earlier on there is a note telling the thieves to search the thieves in the catacombs thoroughly. The gems there can't always be seen, but you can read the texts above the alcoves to find followers of Pjofa, thieves, robbers and people who have stolen things. There you will find 5 gems hidden in the corpses.

That's 18 gems on level 2. (+ 2 from level 1.)

The sling is at level 1 under a barrel before the library. The knoffer is under a cave in to the east when you first arrive at level 1. And there is a knife at the beginning of the game, where you teleport into the main game. An Assasin dagger can be found behind the iron door on level 3 just before the stairs down to level 4.

I have just gotten one blood soaked skull for opening the glass walls (on the many torches area). I am stuck at the auto-circling teleporters part. I think I have pressed every button. The teleporter in the middle corridor is yellow and always takes me out from that area. Ideas?

There are two skulls in each area, if you only got one then you need to go back and find the second one.
The blood soaked skulls in the circling teleporters area are held by the statues. Once you've gotten the skulls you use the yellow teleporter to go back to the glass wall area and teleport out to the next area where you find blood soaked skulls.

I have revealed most of level 7 (air) but, after falling from all the pits to level 8, there is a room with a lot of locks hidden behind liftable walls that I don't know what to do. Ideas?. Also, at an earlier try, I discovered a goromog statue with a plate. I don't know where it is now as it seems I cannot reach it. I don't know what to do with it either.

There are keys hidden on level 7. It's easiest to wait until you've filled all traps because the opening and closing pits stays shut after that. Then look in the wall drains by the floor when standing on the closed trap doors.

I think you're talking about the eastern puzzle where you throw rocks through doors that closes. Once the rock reaches the plate you're teleported to the next part.

I have already close all pits on level 7. And I have also done the eastern puzzle. I haven't checked those drains though.