Do not let the enemies leave the map. You will be informed as to which side they enter from. Set up towers to destroy all of the enemies that try to invade and escape. You will earn money for all of the enemies that you destroy and this will allow you to upgrade your towers or buy additional towers. Tips will be given to you as you go along. As you progress through the levels the enemies will come faster and stronger. As that happens, you will have more evolved towers available to you at a higher expense.