By hacking 13 ctOS towers, you can complete investigations to earn various rewards including achievements or trophies (if you are playing a version of the game that supports them).
The biggest benefit of hacking the towers is that it will cause points of interest to become highlighted on the map, so you can easily conduct rewarding investigations. They appear independent of icons that are tied to any towers you interact with while completing the campaign. If you're looking to complete investigations, you should therefore start by hacking those towers.
To hack a tower, simply head to one of the points on the map indicated by the nasty red icon. Once at the property, work your way around by hacking cameras and unlocking doors (and sometimes doing a little extra, like heading across rooftops or manipulating objects that hide an important camera from sight) until you reach the panel. Stand at it and press the button indicated to finish doing so, and the map will automatically populate with new points of interest. If you unlock all 13 towers, there's also an achievement/trophy waiting as a reward.
The tower in Brandon Docks is a little trickier than most and warrants special mention. You need to find an outbuilding with a box on it, which will open a door on a distant rooftop. There's a container ahead of that rooftop. Drive a car into the container, then ride the container up to the higher level. Drive forward at full speed with the car, flying out of the container and onto the rooftop. Then exit and pass through the gate. From there, climb up the ladder to find the panel you need to hack.
In the Mad Mile area, to tend to the tower in the northeast quadrant, you'll need to start by getting to the top of the large parking garage (not far from the train tracks). Steal a car and drive up to the rooftop. Head around the building and you can find a place where you can drop off to a lower ledge that connects that building to the next one over.
Cross along the ledge, then climb a ladder to the rooftop. You can't open a gate yet, but scan and you'll find a line leading to the right. Head around the building to find the switch console that unlocks the gate. It can't be used yet, though. Keep traveling around the building to find another console, and use that one. It activates the previous console you passed a second ago.
Head back around now and Intrude the console. Then work your magic in the server room you reach as your reward. Now, return to Aiden outside and continue around the building to pass through another gate and use the panel that lets you unlock the tower.
The other boxes are positioned conveniently enough that you should be able to find them out just by exploring various rooftops. Then you're ready to get to work on your investigations.
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