Fortify is Combat Girl's skill which allows her to temporally fortify any allied Combat Kitties, as indicated on the respective turrets by a pattern of 0's and 1's. Doing so increases the range and fire rate of the affected kitties, in a manner similar to the Support's former Hack skill.
Upgrading Fortify decreases the cooldown and increases the turrets' effectiveness even further.
Fortifying Combat Kitties at later levels can activate a slow effect on their target making this perfect for dealing with commandos.
In the past was the skill also able to affect base turrets, turning them into immensely powerful weapons against opposing pros.