Levers
From the first pannel you should now know which position refers to which symbol
Solution:



Levers with a Friend
Simply go forward and figure out which symbol is about you and which symbol is about your new friend


Enter the Abbey
A new friend appears in the south east of the area

What does he want you to do?
Follow him
Keep on following and looking for him until "puzzle" is solved
Locations:


Abbey door puzzle
What is the hierarchy in this world?
Solution:


Trolleys puzzle and coin for the beggar
The key trolley is here, if you manage to move it it's a win:

You can aim for this layout:


Find the right pot




Where to use the key?
There is a door at the right of the garden

Where to use the lens?
Does this shape look familiar?
It's the key to the mini puzzles in the abbey
simply put the lens in the shape and it gives you the symbol to choose

God is watching
In which position should be those statues?
You got a big clue from the abbey:

Solution:

Where to start?
There is a hole at the right of the entrance:


Build a bridge
You need a tool
There is a spear in the previous room


Pass the guards
Here I won't explain how to precisely move in each room as it's linked to timing and a bit of skill. Just try some steps and from trial and error you should reach your goals.
In this area:

you need to get this box in order to make a bigger wall to hide in the elevator platform:



Disguise
The right disguise is sword / long helmet / armbands
From there you can move much more easily in the fortress, congrats!


The Observatory
Memorize the shape here:


Follow the orders
As the title say in this area simply follow the orders:


Where is the light?
In the left of the room you should be able to pick up the lignt to use it downstairs:

This pillar can rotate:

and this is your goal position:


Treasure room
Which object to choose?
Remember the shape of the lens in the observatory?
You must pick the lens in the 2nd lign / 2nd column:

You know where to use it :)

Need a drink

Where can you find a bottle?
You should go back to this place to find it:

It doesn't do anything? Try to leave and come back

The bells
What is this book about?
Which song to pick?
This fortress is all about the chosen
The right tune is the fourth
Solution: balance, instrument, key, vessel, key:

The monkeys

How can you feed them?
What does it usually eat?
You can pick fruits in the trees around:


Go downstairs
Where does this woman go?

You can simply follow here

The right tool
You must pick the hammer:


The mill
Simply follow the path in order to get the torch

You know where to put this torch if you explored well, there is a door on the west of the area with the shape of the torch in it

Mini games
First game is Flappy Bird so maintain the button to make the bird fly through the holes
Second game is a "find the intruder" game, if you hit the grey blue face each round you should be able to fill the gauge at the top, it may take some time as it's a rapid game but eventually you'll get your token

Look at the symbol on the token
It's your entrance to the theater!

The theater
you got the token to enter from the mini games at the east of the area

The sewers
You have to reach this room under the agora:

There you have to pick something which fell in the sewers
It's a compass:


The "labyrinth"
Remember the theater play? and the book with directions in it?
You must follow west, north, east, south, west
In order to accomplish this at each step you should put the red arrow of the compass on the north like this, and then go the direction you want:

The monster
The monster seems to dislike light
Use the levers in the level to bring some light

Elevator machine
What are theses stickers for?

Here is the right order:


The restaurant
At what time do we eat?
There is a clock on the left[/spoiler] look at it closely
You have to wait for the clock to be on a fork symbol and follow the scientists inside:

Inside you'll find silver cutlery

Access to the mines
You can jump in the trolley right after activating it:

There you'll be able to use this machine to melt the cutlery:

Here is how it works:

Yeah you get silver!
You can also get gold from a room nearby:

In the locker you find punched cards
Where can you use these "access" cards?
It can be used in the terminals next to doors, with a red light above and a card slot

Lighter
After using your punched cards you save a scientist, there you can find a lighter:

Did you notice a candle before?
You can use the lighter here:


Door code
The code can be found in the library:

It's cystercian system and here is the answer:


Forge the key
Behind the door code you find a "solution":

Zoomed version:

As mentionned before this area use cystercian system for numbers, so the number at the top left is the thousands digit, the top right is the hundreds digit, the bottom left is the tens digit and the bottom right is the ones digit, leading to this:

You now need to go to the forge room and adjust the weights accordingly to these values, here are the 3 answers: