The word 'portal' is not often associated with gardens - people normally think of portals as an outer space term ( i.e. entering a worm hole) or an internet word referring to an entry point to other websites.Here this lovely grassed path on the grounds of the wonderful Ashintully Gardens in Tyringham, Massachusetts (click on link) has no specified portal, although I love the two stone round finials on either side of the path entrance....
Garden makers are becoming more cognizant of portals and how they can contribute to the overall experience in a garden - the current popularity of wooden arched arbors and gates in my part of the globe is testament to this fact.
But, as in all design, it is about context and the presence of an arched entryway does not a garden make! Be mindful of where you place your gateway and make sure the threshold beneath, in front and behind, supports a vertical element. I am talking here about proportion and scale. Therefore, if your arched gate is 8 feet high, make sure that the area extending 4 feet in front of the arbor and 4 feet behind the arbor is part of the entry experience. It can be more than 4 feet but make sure it is at least half the height of the arch.
In the arbor above, I designed the bluestone steps and cheek walls to extend out as far as the gates. This creates a welcoming doorway and is in proportion to the height of the arbor! The Goldflame Honeysuckle climbing up on arbor's lattice side also helps....
Of course portals do not have to be so grand. Here just a few gate posts signify an entry. This is a photo of the gate to the house and garden where my favorite garden writer lived. I don't know whether May Sarton designed this fence and gate - but nonetheless, it is a lovely example of a quiet but effective garden portal.
above garden with stone wall, steps and white arbor by Johnsen Landscapes & Pools (click link)

