Your front archway is part of the perimeter fence, and consequently it deserves a mezuzah, without a berachah.
But the archways over the garden path are not within a structure and are therefore exempt according to many authorities. Other authorities recommend that even free-standing arches deserve a mezuzah, also without a berachah.
However, your garden path archways are exempt for another reason: They are purely decorative. As such, even if they would be in the rectangular form of a normal doorway, and even if they would be indoors, they would not need a mezuzah. An example of this would be a long hallway that an architect has designed with repeated doorways for purely aesthetic rather than utilitarian purposes.[A] Another example would be a decorative archway, inside a room, that serves no real function[B]
On the same topic, if the a room is enterable by multiple archways that stand side by side,[C] they are considered one collective entrance, and only one mezuzah would be necessary, which would be placed on the far right side.
 Kuntres HaMezuzah 287:16; Pitchei She’arim 287:52. Shevet HaLevi 2:157.
 Agur B’ohalecha 22:15:30, citing Chazon Ish Y.D. 170:2.
 Shulchan Aruch 286:21; Chovas HaDar 2:19; Shevet HaLevi 5:160; Agur B’ohalecha 23:24.
 Shulchan Aruch ibid.