Hidden areas | Crestwood
Dragon Age: Inquisition Guide
The above map presents locations that are more difficult to access, which you can visit while exploring Crestwood.
The passage to the dam in Caer Bronach
Point 1 on the map shows the door to the dam in Caer Bronach. You get there while completing the "Still Waters" side quest. Interacting with the dam's mechanism lowers the water level in the lake and grants you access to the unique locations mentioned below.
Point 2 on the map is the Old Crestwood. By default, the area is inaccessible, because the village is, almost entirely, under the water. The only way to gain access to the Old Crestwood is lower the water level in the lake (the "Still Waters" quest) and it has been described in description for point 1.
The entrance to the Flooded Caves
Point 3 on the map marks the entrance to the Flooded Caves, i.e. the additional underground location. Just like in the case of the Old Crestwood, the only way to access it is by lowering the water level.