The arcs in the arbor and the circle in the trellis are probably the most difficult. Each were made by joining angled sections of 8" lumber roughly in the particular shape. I used a Festool Domino to cut the tenons to join the pieces together. I then glued the sections with the tenons with Tite-Bond glue. To get the final shape, I made a pattern of a half circle out of 1/2 " MDF, and fastened it over the glued sections, and then used it as a template to cut and fine-tune the curves with a router in incremental passes. For the circle, you simply route the top half and then flip the template in order to route the bottom half. Dado 3/4" stock and then rip it for the lattice rather than purchase the thin stuff at a home center.
Every garden and landscape needs a little something vertical. The key is positioning the arbor or trellis in a good, sunny position, if you intend to grow roses. You also have to make an expense decision on whether to use cedar or simply KD & PT lumber.