The Fallen Watchtower is a point of interest in the Watchtower Cliffs. Sitting high on the cliffs overlooking the lake, this ruin is used by the local centaur forces to conduct rituals as well as to store large reserves of Bloodstone materials.
Go to the Peach Tree point of interest, then leap and glide among some ledges to exit the cave to the east. Run up the grassy slope to the right, then turn north and climb up the diagonal rubble pile. (If you don't have a Springer start from the northmost point where the rubble touches the rock wall). After you climbed up, just dash through all the centaurs into the structure surrounding the PoI.