You are left alone to defend the base and this has come to the knowledge of the enemies. They use the knowledge to gain some tactical advantage. Now, they have surrounded the base thinking that there would be no one to defend it. However, you are a soldier that doesn't know the meaning of afraid. You are the bravest soldier of them all and you are left alone to defend the gate. Basically, the idea here is to defend the base against an all out attack. You need to eliminate them all. You would gain some money for killing enemies and you can use that to repair the base or buy a better weapon.