The four corners of the map (World Map, not Level Map) are clickable once you open hextiles close enough to them. Each one has a numeric value, as though they were numbers on the numeric keypad (shown to left). If you enter a specific series of value, you can enable a secret, usually cosmetic, enhancement.
|11331791||Grey Trees (finish a level with this mod for an achievement)|
|13371337||Spinning tower shots|
|77311973||Gift gems (level 1 gem at start; always selected from the field's original available gem types)|