Successfully complete the indicated mission with a gold medal to unlock the corresponding bonus map:

Best Defense: Mission 17
Check Mate: Mission 8
Construction Junction: Mission 6
Monkey Troop: Mission 5
The Blue Dragon: Mission 13
The Great Wall Brawl: Mission 11
The Question Of Faith: Mission 19
The Ship Of Fools: Mission 14
Tower Of Power: Mission 16
Under Construction: Mission 3