You should never let your ACU nor your Engineers sit idle for even a second, as it can easily cost you the entire battle.
As a note, all of this building should be taking place simultaneously. This area is located just below the Rapid Fire Point Defenses you started constructing already. Meanwhile, your other Engineer should be building High Damage Anti-Air Towers on your stronghold's east side, right by the north-most Energy Generator. Though you'll want to put up two to three at first, your goal is five or six, wrapped around the exterior of the Extractor. These will be the hardest to maintain, and will require constant supervision to ensure they aren't eradicated. During this time, have one of your Engineers start building a similar group of anti-tank turrets in the northwest, right on top of the Mass Extractor located there.
