The Medium Turret is a Turret that is available in Tech Level 3 and costs 25 Carbon and 125 Ore to construct. It is a stronger version of the Light Turret. In Tech Level 4, it can be upgraded to the Advanced Turret at the Research Center for 200 Carbon and 500 Ore.
Like with Light Turrets, Workers and Troopers can garrison inside Medium Turrets to protect themselves and increase the Turret's attack strength. Medium Turrets cannot attack aircraft, and siege units (like the Pummel and Cannon) can take them down in a matter of seconds.
"At Tech Level 3, you may use your Research Centre to upgrade Light Turrets to Medium Turrets, which gain appreciable bonuses to their durability, range, and attack power, and may be used to garrison Troopers."