For many levels the enemies come from "off screen" so there is no base to attack. There are some levels where you are supposed to send troops to destroy the ene

For many levels the enemies come from "off screen" so there is no base to attack. There are some levels where you are supposed to send troops to destroy the enemy base, but once you beat that level if your base there comes under attack the enemies come from "off screen" again from what I remember.