In the third Alliance mission (where the elves are escaping from prison) there is a room with three sheep near the middle of the map. When a unit steps on a panel near a sheep, the sheep will say one of three things. Step on the panels to say "Baw, Ram, Ewe" to open a door above them. The door leads to a bunch of flying sheep and a big question mark. Pick up the question mark as if it were an item to unlock a secret tower level.