Make sure that you have Menge’s Key in your inventory:

You need Menge’s key to find another way out of the witch hunter’s outpost in Count Reuven’s Treasure in Witcher 3 Wild Hunt.
Then open the door in Menge’s room that leads outside:

Go through this door to find another way out of the witch hunter’s outpost in Count Reuven’s Treasure in Witcher 3 Wild Hunt.
Down in the courtyard go around the back of a building and climb onto it:

Climb over this building to find another way out of the witch hunter’s outpost in Count Reuven’s Treasure in Witcher 3 Wild Hunt.
Drop down on the other side of the wall.
and how to get out of thet circule?