The Advanced Turret is a Turret that is available in Tech Level 4 and costs 25 Carbon and 125 Ore to construct. It is a stronger version of the Medium Turret, and is the strongest anti-ground turret available in the game.
Like with Light and Medium Turrets, Workers and Troopers can garrison inside Advanced Turrets to protect themselves and increase the Turret's attack strength. Advanced Turrets cannot attack aircraft, and siege units (like the Pummel and Cannon) can take them down in a matter of seconds.
Upgrading Medium Turrets to Advanced Turrets is a good improvement, but Ion Accu-Accelerator is more important and should be researched first, as it will let the Turrets do far more damage.
"At Tech Level 4, Research Centres may be used to upgrade Medium Turrets to Advanced Turrets, which are even stronger and tougher, and are especially excellent for garrisoning Troopers."