The Watchtower is a location at the southern part of the island.
After you discover that place, you will receive the quest "Explore the Watchtower".
The entrance to The Watchtower is locked, you can unlock it from the other side.
Parkour and entrance
There are some zones with parkour in The Watchtower that you can use it to climb the tower, when you reach the top, you can finally enter to it.
At the bottom of The Watchtower, there is a kidnaped guy that you can talk with him.