Wait for different times to see if the door unlocks with a change in daylight or nightfall.
Keep an eye out for enemies dropping keys or purchase them from merchants. Certain quests also offer keys as rewards.
Pay attention to quests that offer unique keys. For instance, completing the "Tolled to Rest" quest rewards you with the Ancient Battleground Key.
Utilize vocation abilities like climbing walls or roofs with classes such as Sorcerer, Mage, or Fighter
Not all locked doors need keys; sometimes, there's a hidden passage or climbable wall nearby.
Some areas may remain locked until you progress further in the story. Advancing through the game may naturally unlock inaccessible areas.
Your pawns may offer valuable hints if you're stuck. Don't hesitate to consult them for guidance.