A domed entrance:
An entrance that has two doorposts with a dome shaped lintel over it, requires a Mezuzah if the doorposts are both at least a height of ten Tefach. This length is measured from the ground until the point that the doorposts begin to curve. If the doorpost is less than ten Tefach high before it begins to curve into a dome, then a Mezuzah is not required.
Placing the Mezuzah in the curved area: When the curve of the dome extends for a great amount of height above the ten Tefach doorposts, and thus the top third of the entrance is by the curved area, the Mezuzah is to be placed by the curved area.
Exhibit d: Exhibit e:
 Taz 287/2; as rules Rambam and Michaber
Other opinions: According to Rashi even if there are no actual doorposts and the entire entrance is a curving dome [Exhibit e] then its obligated to have a Mezuzah if there is a width of four tefach to the entrance and it reaches a height of ten teach. Thus according to Rashi the curving area of the doorpost is also included in the measurements.
 Michaber ibid
The reason: As it is considered to be lacking doorposts.
 Taz 287/2
 Which is the area which the Mezuzah is to be placed on, as will be explained in chapter ??.
 The reason: Although the curved area is not considered as being part of the doorpost, one should nevertheless place the Mezuzah in that area, as although it doesn’t initially count as part of the doorpost, nevertheless it can be considered an extension of an already existing doorpost. [Meaning even according to the Rambam/Sh”a this would be valid. As according to Rashi being that the curved part is considered part of the doorpost, this is its rightful area.]