There was an awful thing, started the third world. You have to protect the interests of the defense of the country. You will not attack the enemy, all you have to do, do not miss it on your territory. You provided all border money and few people. Build and improve turrets. The enemy will attack from two fronts: on the ground and the air. Sometimes it is wise to improve the old building than to buy a new one. Properly dispose of the funds.