Reposting this from /r/Gemcraft. These are a collection of the discovered secrets in Gemcraft: Chasing Shadows. Some of this may be slightly inaccurate, but hopefully compiling all the knowledge here will get us closer to the final answer.
Grey Trees
ReaperGuy on the AG forums figured out GreyTrees, and Seberoth also identified some extra codes. It was discovered that the outside corner squares of the map (zoom out to make them easier to see) are clickable. Various codes inputted will do changes to the game, but so far it's only cosmetic aside from the almost-useless Gift Gems.
The codes are entered with the following scheme (as if they're four corners of a number pad): Southwest = 1 Southeast = 3 Northwest = 7 Northeast = 9
The codes discovered so far are: 11331791 - Grey Trees - To get the achievement, finish any field with this set. 11319773 - Black Orb 13371337 - Spinning tower shots 11379197 - Mossy towers 97713791 - Mossy amplifiers 77919713 - Heavy rain 79797919 - Heavy snow 77311973 - Gift gems 11799397 - Dark monsters 11111111 or any 8 digits of the same number - Mods reset
Assuming up to 8 digits in any given code, there are still over 65k codes to try. I don't think brute forcing these will work. Again, all credit for this discovery goes to ReaperGuy and Seberoth on the AG forums.
Compass
The basic gist is that you need to set all compass pieces to tier 5 and beat the final one that drops the field marker. Rinse and repeat 11 times for talismans and field markers. The third reward is a bugged field marker that prevents the round from finishing. Just manually end it and start on the next compass set. You'll know you have completed them all when the gems no longer show up in the loading screen.
The compasses themselves correspond to the gem tiers you see on the loading screen of compass levels. Teardrop is tier 1, triangle is tier 2, square is tier 3, a "U" shape is tier 4, and an "X" shape is tier 5. Every time the compass tiers reset, the compass correspondences are set to a random orientation, which always follows a pattern of 1-2-3-4-5-4-3-2.
Let me be perfectly clear: The directions on the compass don't matter for any tutorial/description, because where Tier 5 is pointing is random each time you do things such as reload the game, change your skills, or beat a level.
The compass levels, in left-to-right order, are C2, E3, J2, K2, O3, U4, and X7. The easiest level to finish at is E3, with only 24 waves at untraited Looming.
Here is my basic algorithm for setting a compass based upon the gem at the loading screen: Tier 1 - Turn 4 times. Done. Tier 2 - Turn 3 times. Recheck. Tier 3 - Turn 2 times. Recheck. Tier 4 - Turn 7 times. Recheck. Tier 5 - Done (duh).
With this algorithm, tier 2 and 4 always become a tier 3 or 5. Tier 3 always becomes a tier 1 or 5.
The rewards for completion of a certain amount of cycles is as follows: 1 - Field D6 3 - Bugged Field Marker (perhaps supposed to be N7?) 5 - Field O4 (Already acquired through normal progression) 7 - Field P6 9 - Field Q7 11 - Field X6 2, 4, 6, 8, 10th times: Talisman fragments of ascending quality
Missing Fields
This list is FAR from complete or entirely accurate. These are just the notes I have from field markers I have acquired or seen from other sources.
A6 - Complete A4 on Glaring B6 - Complete the achievement "Still Alive B1" D6, P6, Q7, X6 - See compass explanaition above. Y4 (->Y5->Y6) - Complete all waves on X5 without charging up the gem. G4 - Complete the rest of Hex G on Haunted difficulty L5 - Complete L1, L2, and maybe L4 on Haunted difficulty
No access to field C2=No compass for you. Unless you are extremely lucky and that compass is set to tier 5 at the beginning, in which case you get to unlock ONE reward and that's it.
Yeah, "18" premium fields? Counting the ones unlocked by this method, it's already at 22...
if none mentioned it so far, you can unlock U7 by completing U6 with haunting difficulty (not sure if you can unlock it somehow else, completed it only in looming before i attended haunting)
