Turrets are a type of mod that increases a portal’s attack rate and hit bonus when it attacks an enemy agent. Only rare turrets are currently available. Each turret you deploy on a portal increases its hit bonus linearly, but the increase to the strike factor for each turret after the first is less effective:

Mod 1 Mod 2 Mod 3 Mod 4 Hit
+0% ×1
Rare +20% ×1.5
Rare Rare +40% ×1.9
Rare Rare Rare +60% ×2.1
Rare Rare Rare Rare +80% ×2.2

Turrets also have a stickiness value of 200,000, making them a bit more resistant to critical hits than rare portal shields.

Note: On 13 May 2016, turrets were modified to be stronger. The hit bonus was reduced slightly, but was made to increase linearly instead of becoming less efficient with stacking. An attribute called “attack multiplier” was removed and replaced with “strike factor;” it is uncertain at this time how these differ.