There are a total of 13 ctOS Towers in Watch Dogs. Finding all of them will reveal all collectible locations on your map and earn you the Clear Signals Trophy / Achievement. Each tower also unlocks a fast travel point.

You must hack ctOS systems to make the towers available. ctOS systems are always marked on your map from the beginning on. You can either hack them in free roam or wait until the story requires you to do so (you have to hack all of them to make it through the story anyway).
Once you have hacked the ctOS systems you can activate the towers. They are all on rooftops, so it requires some climbing and you must use your hacking skills. Use surveillance cameras to find little control boxes that open locked doors.

None of the towers are missable and you can still get them in free roam after beating the story. Because they reveal all collectibles and additional fast travel points you should get them as early as possible.

TIMELINE:
#1 - Parker Square - 0:05
#2 - Parker Square - 1:27
#3 - Mad Mile - 2:11
#4 - Mad Mile - 3:26
#5 - Mad Mile - 4:36
#6 - The Loop - 6:19
#7 - The Loop - 7:17
#8 - The Loop - 8:25
#9 - Brandon DOcks - 9:38
#10 - Brandon Docks - 10:58
#11 - Brandon Docks - 11:52
#12 - The Wards - 13:08
#13 - The Wards - 14:28