Unlock the following bonuses by performing the corresponding tasks:
BONUS - HOW TO UNLOCK Third ship - Complete the "Assassin" challenge. Stage 5 - Collect at least four keys from any stage. Stage 6 - Collect at least four keys from any three stages. Stage 7 - Collect at least four keys from stages 1 through 6. Hard difficulty - Complete the game under the Normal difficulty setting. Very Hard difficulty - Complete the game under the Hard difficulty setting.