Unlocking Levels 6 and 12
     To  unlock  levels 6 (Atlantis, 2015) and Level 12 (1502, Italy),
complete these requirements:

Level 12 - Italy, 1502: Get a rank of "Air Virtuoso" 
or higher on Level 11
     Level  6  -  Atlantis,  2015:  Get  100%  in a repair on The Twin
Fighter (or any electronic-based repair mini-game) after Level 5.

Unlockables
     Star Glider: Successfully Complete the Game