Flying Units

[Rex rgis of Ter]

That's because spy's can't attack and aren't captureable. All other units however, are.

 

[Farmer Bobathan]

couldn't you give flying units an extra command fly/land (changing between being a flying unit and being a land unit) so they can attack ground units and fly up to get away, only being affected by ranged people and other flying units. you could also have them automatically be unaffected by terrain when they fly. you could also make an airship thats like a boat but flies over the land and carries troops.