Enter the portal that leads to Yggdrasill's castle. Here, go up the stares to fight him, or go to the room in the left. Go through the door to the northwest, which will lead to a balcony. Then go to the right. Go through the first door in view. You will find yourself on a balcony above the foyer. Continue to the right into another room. Go through the door on the same level to a library for a prism Guard. Then, get out and go down the stairs, and throught the door to the right. Kill the angels to the left for a chest that will appear on the top balcony. Go to the room at the right and get a chest with Energy Tablets. Now go all the way to the left, go up the spiral staircases entering room by room. You will find a chest with Elixir, and the next room having a lone ange. Defeat him for a black chest. Inside is a past stone. At the top, there is a door leading outside.
Make your way through the balcony. There is a chest with an Elemental Guard hidden behind a corner to the left of the Northern Arils. There is a hidden chest with Ninja Sword init in the corner to the right. Go throught the door to the right for another set of spiraliling stairs. Go down to enter some rooms. the first you will have a Spirit Bottle. Get back to the room to the northwest of the lobby, and go up 2 levels, and go through the doors to the right. You will eventually reach a balcony with a chest. Inside is the Future Stone. If you go left from that balcony, you will slowly find a room with a Demon Seal. Go down one floor, and enter the room to the left for a Shield Ring. Get yourself back to the lobby. You will find the Future Stone is shining red lights to the red mat. The red light then goes ontop of a circle, and a block appears. It says something, then 4 blocks appear. One is green, one purple and one white near the portal. A blue block is located to the left. If you go left, a similar thing will happen to the Past Stone. Another portal will appear. Then there are 8 blocks total, each color with their element. Push the black block into the left portal, and you will warp to an island near the top balcony. Push the block off the lower right edge. It will fall down between the 2 bars. Repeat this with the blue block, then the red and last purple. Push the purple block into a hole across the bridge you newly created. Then push blocks into the portal at the right in this order: Yellow, Green, Light Blue, and last, White. The first 3 create the bridge, and the last is used to fill up the hole. The force field in the middle will disappear. Next, open the chest for a Sacred Stone. This allows you to warp all the way back to the Tower of Salvation ruins in Tethe'alla; without having to go through the painstaking task of getting out of Derris-Kharlan!!!!!!!!!!
Thats about it.
Edit: In other words, get the Past and Future Stone, this will let you get the portals and blocks. From here, put them in the specific order mentioned, the shield will fall in the place you teleport to, and this will let you have the Sacred Stone; Allowing you to easlily teleport out to the front of the Tower of Salvation and do the sidequests.