They did choose adequate bedrocks. There is a clear relation between Portuguese ditched enclosures and geology that enables an easier excavation. But making it easier doesn’t mean it was easy.
In fact, to excavate the ditches, that in South Portugal started at least in Late Neolithic (second half of the 4th millennium), they must have used the technology developed since earlier times: the technology developed in mining for flint or for minerals, like variscite.
I have already drawn the attention for the fact that there are several evidences of “transference of technology” in Prehistory, from some areas to others; from some kind of architectures to others. For instance, the building of the access corridor between walls to the inside enclosure of Castro de Santiago is clearly an application of megalithic building procedures.
So, to study the technologies adopted to open ditches it is in mining tools and in mining techniques that we might find a window. There is a clear interesting relation that can be established between the researches of these two Neolithic practices.
And the same can be argued about the hypogeal building tradition. There is knowledge and an assemblage of techniques and tools that must have had a transversal use in different building activities. Underground techniques of excavation are certainly earlier than ditched enclosures in western Iberia. They might have provided the means to the architectural materialization of some new ideas. Just like the megalithic “engineering” was certainly helpful for walled enclosures.
So, looking into mines, and mining material, might be useful when we study ditched enclosures.