Watchtowers are huge towers that can be deactivated / cleared if you kill the Elite Wraith roaming around them. You must do this to unlock additional activities located near the tower. There are 5 Watchtowers in total.
Once you clear all watchtowers available in the game, you will unlock the "Clear Sight" achievement / trophy.
Watchtower (Caladrias)To clear this tower you must defeat Scrimmur.
Southern Watchtower (Monsalar)Clearing this tower is part of the main quest "The City of the Sun". To clear this tower you must defeat Dromur.
Central Watchtower (Monsalar)To clear this tower you must defeat Fractur. To get near this tower use the Forgotten Path.
Northern Watchtower (Monsalar)To clear this tower you must defeat Novur.
Watchtower (Black Alms)