Encircled area is:

(a) Facial recess(Ref. Cummings, 6th ed., 2192)In the given picture, the encircled area is the facial recess through which the round window opening is visible, also see the schematic picture below.Facial recess is used to approach the middle ear through the mastoid, e.g. in cochlear implantation and in intact canal wall surgeries of the mastoid.Schematic picture of facial recess, through which a drill is pointing towards round window: