After researching Lasers, Heavy Lasers, Ion Cannons and Rocket Launchers as far as you can, research Heavy Towers. Surround your base with walls, and at certain intervals, place Heavy Towers. At the top of the Tower place a Heavy Laser and place a normal Laser on top of it. On the left side place an Ion Cannon and on the right side place a Rocket Launcher. The Ion Cannon freezes any unit that gets near and disables its shields, allowing the lasers to destroy it easily. The Rocket Launcher will destroy anything foolish enough to attack from the air. Also use the highest level shield for your structure. For added defense use Artillery. Also do not forget to use Radar emplacements to detect stealth units.