I'm NOT covering achievements nor missiable ones - at all, as there's already a great guide for it by Techim and I won't devaluate his/her work. Go check it out if that's what you're looking for:

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=642992795

Items you can pick up are displayed in bold.
Optional steps you don't have but likely want to do, are displayed in italic.

• Choices that affect your progress are listed.

Disclaimer: I tried to be as thorough as possible, but likely still may have missed parts.

Missing info:
Lucrezia's picture - leading to what effect?
Miss Rat helps how?
Mirror helps how?

You receive 1 cane, 3 gold coins and 1 cheese.
Pick up bone from the barrel, give it to your dog.
Optional:
Re-enter the house, pick up the bread from the table. Exit again.
Top right corner, next to the well, is a tree with an X on it. Check it for 3 chestnuts.
Lower left corner (clockwise all the way around the fence of your house) is a chest with 1 candle.

Exit up and keep heading that way until you see an old woman with a pig.

2 choices:

• Talk with the woman and hand over your cane - you later receive a ring. (preferred action)
  
• Ignore the woman and keep the cane - you're not getting the ring but can later use the cane for 2 shortcuts.

Continue up, right past the raven.
Optional:
At the raven, head up. Top left corner is 1 candle and a J.F. note. Exit down, heading right.

Head around the donkey into the fence, pick up 2 apples.

2 choices:

• Give the donkey 1 apple (don't give him both) - donkey makes it into the castle. (preferred action)
  
• Ignore the donkey and keep the apple - donkey won't show up in the castle.

Heading right until you run into a merchant.

2 choices:

• Talk with the merchant and offer him gold - you lose 2 gold coins, but receive later a letter worth 5 gold coins.
  
• Ignore the merchant and keep the gold coins - you'll later have to buy the letter from him if you want it. (preferred action, as his buyable letter is fake)

Heading right until you run into a shepard.
Optional: top right corner of the tombstone has a chest with 1 candle and J.F.Note.

2 choices:

• Talk with the shepard and offer him cheese - you lose 1 cheese, but receive later a crowbar.
  
• Ignore the shepard, keep the cheese and head up to check out the tombstone - you receive a name from the tombstone, but won't get the crowbar later.

Pick up candle, head down, then keep following the blue guy to enter the castle.
Optional: if you feel brave, you can get 2 more apples at the place where the donkey was. But a goat will attack you.

Note: directions are usually starting from the center of the castle, where the fountain is.
There are two ways to get the name:

• Get enough gold to buy gold key, heading to blue guys room to find it out.
  
• Working your way through the rooms until you have all 10 letters.

-----------------------------

Talk with the merchant to receive blue key. If you gave him gold coins before, you also receive your first letter (which is fake).

Head up. (If you gotten your first letter from the merchant, the blue guy wants to talk with you. If not, it happens after the first room).
Enter the first blue door to your left.

Move up on the right side past the (obvious) traps to the big chest for your (first or second) letter.
Optional: top right corner 1 gold coin, top left corner in the drawer 1 bread.
You can't reach top left corner chest yet, you'll need the wooden plank from the Maze room (green door) - inside is 2 gold coins and J.F.note.
Exit, then left. Take a look at the picture (clue for King room) and grab 3 candles from the drawer.

Head right towards the second blue door.
Optional: enter through the (removed) grate and up to get the mirror and a chest with 1 bread (chest won't be here if you didn't gave cheese to shepard). Exit and enter the blue door.

Move the crates until you see the switch on the upper wall. Move the 4th crate in front of it up, then the 2nd to the left, hit the switch. Get the letter from the big chest to your left.
Optional: top right corner chest next to green door 3 candles and J.F.note.
Chest in top left corner: move the two lower crates down to access the left part. If necessary, exit the room to reset the crates. 2 gold coins and J.F.note waiting in the chest.

Exit, head right and down, talk with Giulia. Receive 10 candles. Exit to the left towards the fountain / center.
Talk with the old woman to receive a green key. If you gave her cane before, you also receive snake ring.
Optional: head back to 2nd blue door (Crate room), enter green door for 2 gold coins and J.F.Note. Back to center.
Head down, then left for the first green door.

On your way to Kings room, you'll find the donkey (if you gave him an apple before). Put a candle down next to him and give him all your apples for 2 gold coins each (may occur later, depending on how many rooms you already visited).

Set the 4 switches into this position: up, up, down, up - head into the open cell, get the bucket and 2 gold coins from the chest.


Now set the 4 switches into this position: (change it from right to left !) up, down, up, up - head into the second open cell, get the letter from the big chest.
Optional: pick up an apple and J.F.note from the chest in the top right corner and 3 candles from the drawer in the top left.
Exit, head back to center.

-----------------------------
It's 3PM, you get teleported to the center. From now on, enemies patrol the corridors (may occur later, depending on how many rooms you visited).
-----------------------------

Back at the center, use the bucket on the fountain to fill it. Exit right.
Enter the green door to your right and slightly up.

Straight up are 2 gold coins and a J.F.note waiting for you. Continue to the right, up and left. Straight up, make it to any of the top corners. Above the big chest, pour the bucket with water into the irrigation system, then pick up the letter.
Optional: top right corner has cheese. Top left corner push the upper barrel to the left, enter the grate.

• If you didn't shared the cheese with the shepard but went for the tombstone, you'll find Miss rat here. Inside the grate, head up, give cheese to Miss rat. Exit lower left.

Exit through the lower left. Back to center.

Head left and enter the green door left and slighly up.

Optional (don't if you didn't shared cheese with shepard): if you followed this guide throughly, light the brazier and go straight back to center. Talk with shepard to receive red key and especially crowbar now, so you don't have to skip 2 gold coins in this room.

• If you still have the cane, use it on the broken switch as a replacement lever. Head around the corner to your right, pick up the letter from the big chest and leave. You can skip the rest of this room.
  
• Without the cane, you'll have to go through the maze as follows:

Go left, 1st up (before Giulia), right and up for 3 candles. Back to Giulia, talking with her for 5 candles.
From Giulia left, up, left, up, place a candle. Go left for a wooden plank. Back to your candle (pick it up).
Continue right and all the way up for 1 gold coin (beware the trap - pick it from the side). To your right the drawer holds 3 candles. Place a candle.
Go down to an intersection. Place a second candle. To your right, up, right is a switch - hit it. Go back to your first candle (next to drawer, pick both candles up again).
From the drawer all the way left and up. Place a candle.
Optional (you need crowbar): to your left, down, left, down, right is a crate next to a goat - open it with crowbar, so skip or go for it if you have it. Inside you'd find 2 gold coins. Back to your candle.
Go all the way right, down, all the way right, up for a bread.
Go down and the first possible down again, left, down, right for the letter in the big chest.
Use switch, then get out and back to the center.

Optional: head back to Trap room (1st blue) and use wooden plank in top left corner for 2 gold coins and J.F.Note. Back to center.

Talk with the shepard to receive the red key (if you haven't already). If you gave him cheese before, you also receive crowbar.

Head down and left to the 2nd door (red), just past the green one.

Hide under table, pick up ladder.
Optional: the bookshelves may give interesting background to the castle and it's residents, but nothing you need.

• If you don't have the crowbar, work your way up to the table in Red Camerlengo room.
  
• With the crowbar, exit then open the grate next to door outside with the crowbar, enter it.
  Head up and left for 2 gold coins, then up, right, use crowbar.

In Red Camerlengo room, hide under the table.
Optional: to the left are 3 chestnuts in the drawer and 2 gold coins in the chest. To the right are 2 bread in the drawer.
Place a candle between table and big chest and go past the big chest on it's right side upward into a back area. Move the crates to the right, to open a passage to the clock.
Optional: pick the 5 candles on a barrel.

Wait until Red Camerlengo opens the big chest, then use the clock and head back through the passage (alternative: quickly hide under the table). Red Camerlengo will go to the clock, use that time to get letter from the big chest.
Then get out and back to the center.

Head right and enter the blue door right and slightly down.

Use the ladder in the back of the room, head up, flip the switch.
Optional: 5 chestnuts and J.F.note in the chest to the right.
Head back down, pick letter from big chest, exit to center.

Head left and enter the red door to your left and slightly down.

• If you have, put the snake ring on. All red chests are fake. 2 are real.
  
• Without the ring, you're screwed and have to check every chest. But that's why you're reading this guide. :)

Top left corner, the topmost big chest on the left wall has the letter.
Optional: top right corner, the topmost big chest on the right wall has 3 gold coins.
3 drawers on upper wall. Left has 2 bread, middle 1 apple, right 3 candles.
Back to center.

Head down and enter the red door to your right.

Invisible traps and servant - avoid the blood stains on the floor (traps) and listen to the whisper (servant).
On the upper part of this room are 3 switches, left, middle and right. These are your first goal.
Right one reveals the servant.
Middle one reveals traps.
Left one reveals the braziers.
Optional: upper left chest on the left wall 2 gold coins. Chest directly to the right from previous one contains 3 chestnuts. Chest directly up from entrance 1 bread. Lower right corner chest 1 apple and J.F.note

Light all 6 braziers, then open the chest. Exit.

Head down, talk with Giulia to receive numbered piece of paper. Head to center, use it on fountain - this shows the position of each letter used in the kobolds name. If you don't remember, letters are written on the doors of each completed room.

Talk with Eisbein (the pig), then talk with shepard. You have 3 options:

• Use goat to kill the old woman
  
• Use goat to kill Eisbein
  
• set trap for Eisbein (preferred action)

Buy corn, use it on Eisbein. Head right to cage room (right and slightly down, blue door). Use corn near chest.
Back to center talking with Eisbein, then back to cage room. Head up, flip the switch, get letter from Eisbein. Doesn't seem to matter if you free Eisbein or not. Exit.

If you collected all J.F.notes in the castle rooms, head back down to Giulia to learn about Lucrezia's treasure, head to the Library. Get the book from the leftmost shelve in the middle of the Library, then back to Giulia.
Give her the book to receive code, back to center, use code on chest to receive butterfly net.

Numbered 1-4 from left to right. A lit brazier in front of the door means someone is inside the room.

Room 1
Cabinet holds 1 gold coin, drawer near bed 1 missive.
Room 2
Drawer holds 2 cheese, 1 missive on table.
Room 3
Cabinet holds 2 gold coins and 1 missive

You want this situation in the center area, if you want the kobold to spoil his name (two guards facing clock, rightmost window lit):






Room 4
If the brazier is lit, enter, then hide behind the curtain and wait until he reveals that his name is Patronio.
Top left drawer 1 gold coin, drawer next to bed 2 candles, drawer to the right 3 candles.

Head right, then up (don't use bloody chest) and right again. Talk with J.F. and receive 1 gold coin.
Optional: 2 bread from the chest.
Head left.

There are 3 cave entrances here. Left, upper left and up.
Optional: lower left corner 1 cheese

Optional: short before the cave, are 3 candles to your left.
Enter the cave. Receive worm.

• If available, use cane on switch, get golden emblem, head back out.
  
• If not, go through as follows:

Up, at the first intersection, you'll need to flip that switch to your right.

• Either pass those 2 traps to your right, flip switch and head back.
  
• Or pass the firetraps and monster by going up, right, down around the corner, flip switch, head back.

Once you gotten that, go further up and use those passages left and right to shake off the monster if necessary. Flip the switch on upper wall.
Optional: at second intersection up, to 2 gold coins to your left. Further up 2 bread on your right and 3 candlesin upper chest.

Head all the way back down. At the upper intersection, go left, down, left, down, right, down as those fires aren't burning anymore. Pick up the golden emblem.

Head back to J.F. using the golden emblem to the left of him. Talk with J.F. to receive seed. Left and up, planting the seed. Down again.

Head left, then upper left to get to the cave there.

Optional: 3 candles to the right and short before cave.
Get a blood bucket from the well behind the house.
Optional: 1 goid coin in upper right.
Head back towards J.F.

You can completely skip this cave, as the golden emblem is a fake one - your choice.
Optional: short before the cave, are 3 candles to your left.
Up, then left, hit switch.

• If available, use crowbar on grate, head in, up, right, down, use crowbar.
  
• Or head all the way right.

Optional: 2 apples from chest.
Hit switch, then left and all the way up hitting the last switch.
Optional: 1 gold coin next to switch.
----

Use the blood bucket on the plant, then go talk with J.F. to receive rusty key.

• Skip it and head straight up the plant - possible endings: Kobold's Trick, Pact with Kobold, Happy but poor
  
• Or head back to your house in upper left cave - possible endings: Wealth without Answers, A Prosperous Future

Enter the house. Don't talk to your mom, don't eat from the cake. Place the worm on the cake.
Head straight after the butterfly and catch it to receive butterfly-shaped jewel.
Head to the plant and get out.

Just sit on the throne.

In the throne room, use the snake ring, then talk a bit and still sit on the throne.

Don't sit on the throne, instead push it aside and exit through the door.
Head straight home to your mom.

Having the butterfly-shaped jewel, don't use the snake ring, don't sit on the throne, instead push it aside and exit through the door.
Head back to where you first saw the raven on a rock, then up. Use the butterfly on the door. Pick up treasures, exit and now head home to your mom.

Having the butterfly-shaped jewel, use the snake ring, talk with the Kobold again, don't sit on the throne, instead push it aside and exit through the door.
Head back to where you first saw the raven on a rock, then up. Use the butterfly on the door. Pick up treasures, exit and now head home to your mom.