Make your way to the large waterfall area and go through the doorway inside the tree at the top. Once you enter it, bounce upwards on the mushroom tops to reach the platform where the fox is jumping over the sleeping dog--for reference, this is also where the Rosetta Stone for the alphabet is found. Head up to the next platform and pop open the chest there to nab yourself the Writing Artifact.
To get yourself this artifact, you’ll need to first head to the observatory area. Once there, make your way halfway up and go through the purple door that was almost directly above from where you entered the area. Once you go through it, you'll reach the area with a temple at the top. Make your way up to it and enter the front door below the three squares. Once through the door, hop up the platforms to get to the top of the book shelf and spin the perspective to see the door below and the green lamp across from it. Hop down to the door and go through to find the chest that the artifact is held in.
To find this artifact, you’ll need to find the four treasure maps that show the proper doors that you'll need to enter to reach it, as well as a key to be able to access it. Head to the cemetery and go through the door that is located to the right of the warp gate. When you reach the next area, go through the locked door at the top of the crypt and you will come to a room with many, many doors. You will then need to use the corresponding maps to figure out which doors in which order to go through to reach the room with the artifact... or you can just watch the video above to see the route to take. Once you enter the room with a tree and red water, jump in the water and swim to the right to find the chest. Open it up and the Skull Artifact is yours.
To get yourself this artifact, you’ll need to head to the clock tower and make your way halfway up, then go through the purple door that leads to the building with the yellow pyramid on top. Once you reach it, enter the front door and you will come to a small room. Spin it around to find a bookcase and press X/SQUARE while standing in front of it. Enter the secret room to find a globe at the top of the room. Use the lever below the globe to spin it around and reveal the door inside. Enter it and you will find a chest that houses the Tome Artifact.
The first piece of the heart cube can be found in the observatory. While looking through the telescope, in one section of the sky you can see a blinking red dot at night time in the upper left hand corner, which flashes with a repeating pattern of: L, L, R, L, R, L, R, L, L, L, R, L, L, R, R, L. Enter this sequence and a part of the red heart cube will appear for you.
For the second part of the red heart cube, when you lower the water level at the waterfall, go through the door on the right and you will enter a room with eight blocks. Arrange the blocks in order to spell out “Metatron” the same way as in the picture above. To get the blocks in the direction you want, turn the perspective until the block matches and then pick it up and spin the perspective back--the block will stay the same as long as you are holding it. Once it is spelled out, rotate the camera twice and the red cube should appear. If it doesn’t, switch around the two “T” blocks and then rotate the perspective twice again.
This third red heart piece can be found in the black block area where the candles were lit and the burnt map matched the symbol on the ground. Look on the ground in first person mode and stand on the opposite square that you stood on previously to make the block appear. When you are centred on the square, enter the sequence: DOWN, DOWN, L, R, R, JUMP, UP and the third heart piece will appear. The collective and incredible brains of the internet can be thanked for figuring this one out by random button presses, but the actual in-game method used to figure it out is still a mystery.
Once you have all three pieces, you can take them to the lone tree area, just off the bell tower area, and enter the door that takes 64 cubes to put them all together as one.