Crestwood is one of the bigger location of the game. In theory, you explore is only during one of the main quests of the game (quest "Here Lies the Abyss"), but this is a short-lasting one. Fortunately, also in this location, there are many side quests to complete and many valuable treasures to obtain. Therefore, I recommend that you spend here some more time. Still, it should be pointed out that some of the fights with monsters and other opponents (e.g. bandits from the fort) require a well, experienced party.
The way to unlock this location: On the war map, select the "Find the Warden" operation and complete it, by spending 8 Power points.
Recommended experience: If you are planning on a short visit to Crestwood only, during one of the main quests of the game, you do not need to prepare in any specific way. If you want to complete side quests also, I recommend that you reach experience level 11-12, as the party. If you want to "clear" the entire map, progress with your party to level 13-14, at least. Thanks to this, you will not have any problems taking Caer Bronach, or defeating the wyvern in one of the side quests.
1) The element that makes Crestwood different from, e.g. the Hinterlands, is the option to seize Caer Bronach. By default, the fort is being occupied by bandits, but thanks to completing the "Capturing Caer Bronach" side quest, it may be seized by the Inquisition and become its foothold.
2) In the central part of the map, there is the titular Crestwood village, where you can meet the mayor, among others and take several optional quests. In the North-Western corner of the map, you can find the Old Crestwood (and the entrance to the Flooded Caves), but it is flooded at first and you cannot explore it. The situation changes after you complete the "Still Waters" quest. To learn more, see the Hidden locations page.
3) While exploring Crestwood, you find a cave with frozen door. Do not worry if even your rogue fails to open it. You will be able to explore the area behind the door, on your way back from the Flooded Caves.