When a curved archway is surrounded at its top and sides by a wall or a fence, even though it lacks differentiated posts and lintel, we conceptually “carve out” virtual doorposts and a virtual lintel from the surrounding structure. Even then, its obligation is doubtful, and we place the mezuzah without a berachah.
To calculate the top third of the doorway, you need to measure from the floor to the point at which the two sides of the arch come within four tefachim of each other. This is the halachic “top” of the doorway because the space above that point is too narrow to be considered a serviceable entrance.
Obviously, in this case, there is no choice but to place the mezuzah on the curved area.
 Chovas HaDar 7:14; Kuntres HaMezuzah 287:15.