After researching AA Plasma Cannons, Plasma Cannons, and Heavy Plasma Cannons as far as they will go, research Fortresses. If your base is in an open area, use some walls to protect it. Place a Fortress into an easily defensible location near your base, such as a valley between two cliffs or a bridgehead. After that, place a Heavy Plasma Cannon, two AA Plasma Cannons, and a normal Plasma Cannon on the fortress and use the highest level shield available. The Heavy Plasma Cannon and the normal Plasma Cannon should eradicate any ground forces and the AA Plasma Cannons will take care of air units. Also, the Fortress has enough armor to withstand plenty of heavy attacks without dying. For added defense use Artillery. Also be sure to place an SDI when fighting against the ED.