(banana?) ‘circles’ – why limit the function with shape? The term banana circle was first revealed in Bill Mollison’s Designers’ Manual. I can’t understand though, why these wonderful structures continue to be limited by a single shape in the name. I suggest that ‘sponge’ (the term that I use) is more appropriate as it describes the function better: a structure which holds water and nutrient over a longer period, which is available as required by the plants (certainly way beyond bananas of course). A circle may be fine on flat land, but as soon as we create them on slopes, elongating them will serve more functionality, as a mini-swale or u-bund. Or we can wrap it around a corner of a structure, or squeeze it into that angle of the garden. 
Also, to suggest that it need be a metre deep is confusing, especially since perhaps the single most common failing in creating ‘sponges’ is not to have enough organic material to really fill them up, which totally compromises the utility of them as autonomous growing systems. As deep as the material is available is a better rule of thumb. And certainly they are not limited to bananas or other tropical species. Especially in these times of increasing dry times, and increasing thunderstorm excesses everywhere; simply choose the species according to the microclimates represented in sponges
What a wonderful teaching tool too, demonstrating all the Pc principles in one structure! . This video is a light-hearted explanation.