All the pyramids and megalithic sites are located on the mountain ranges of the Rhodope, the Middle Mountain, the Central Balkan and the ripples in the relief that lay North of the Central Balkan mountain range. There are no megalithic sites in the lowlands of the Thracian and Danube plain.
It is as if the pyramid builders lived in a world where water levels were much higher than today. We see this in the erosion patterns too. The megalithic sites that are lower around, 200-300 metres elevation and closer to the Black Sea basin are more heavily eroded by water.
As Rudolf Steiner tells us from his account of the Akasha Chronicle as related in the series of lectures: Cosmic Memory, Prehistory of Earth and Man, the Lemurian and early-Atlantean lived in a world that is very different from what we know today.
One must imagine that with the passage of time all conditions on our earth have changed very much. […] air which envelops the earth was much denser […] water on the whole earth was much thinner than today. p.44
Air and water density were different, and maybe gravity too. Matter did not have the same density.
Geological conditions were more unstable with regular volcanic activity.
For Lemuria was turbulent. After all, the earth at that time did not have its later density. The thin ground was everywhere undermined by volcanic forces which broke forth in smaller or larger streams. Mighty volcanoes existed almost everywhere and developed a continuous destructive activity. p.80
And the sea level was probably higher than today, maybe by a few hundred metres. And the land was probably prone to huge flood events like mega-tsunamis. As a result, the pyramid builders built where they could.
They lived where nature gave them the opportunity to do so. p.72