It took hours… and hours… and hours digging in the dust. And the garden looked terrible for months.
as I wrote before, a rectangle was not a possible shape, due to the cypress and the oleander. I did not want to build a wall that twists, it would have looked heavy. I chose to use wooden panels. I found some in the store, Bangkirai wood, that were square, 1 meter wide. They were designed for terraces. I doubled them with chipboard panels. I hesitated whether to place them vertically or horizontally. Actually I changed my mind. The vertical position was more pleasant aesthetically, but the horizontal position was stronger. I put some thick irons between the two layers, and poured concrete. I placed an additional (cheaper) panel inside, on strategic joints. I added iron bars on top of panels, and a lot of thick metal plates, until it seemed to me that the structure was strong enough to resist the pressure of the water. I also added some dirt in the corners, to have a more gradual pressure on a round shape.