There seems to be several of these structures in SW Iberia but many are under urbanized areas. However the one at Perdigoes (Reguengos de Monsaraz) is not and is hence being researched by the University of Malaga.
They are made up of concentric ditches that, for what has been discovered so far, were created sequentially, with the ones at the interior being filled when the exterior ones were dug. The different texture of the fillings, which retain water better, allows for these to be seen in air photographs after rains (photo below).
In Perdigoes, the interior (oldest) rings seem to be dated to the second half of the fourth milennium (c. 3500-3000 BCE), while the most recent are from well into the third milennium. They have found varied stuff in them, including human and animal remains.
We believe that these were spaces that hosted scattered groups for ceremonial purposes such as marriages or funerals. When these ceremonies were over, they made the enclosure to disappear, throwing everything into the ditches.
Source: Terra Antiquae (in Spanish). It also includes a link to a PDF (also in Spanish) on the maybe related mysterious and ubiquitous Chalcolithic silos/wells of this area, which are argued to be local manifestations of more widespread practices of the Megalithic area. See also this news item (ADN, in Spanish too).