It is the end of the line. There is no way out. The only option is to make a stand and fight. The enemy soldiers come at you in waves and you must shoot them all down and survive to tell about it. Use your mouse to aim your rifle and left-click to fire. Your weapon will reload automatically when it is empty. At the end of each wave you will have an opportunity to purchase weapon upgrades and ammunition with the money you have earned. When you are fighting, try for head-shots in order to cause the most damage to your attackers. Keep tabs on your health and ammunition supply by watching the stats in the top left. Good luck soldier and try your best to get out of this one alive.