Note: Dating back to the 5th millenium BC, these Neolithic rondels are circular (or semi-circular) ditched enclosures "mostly confined to the Elbe and Danube basins, in modern-day Germany, Austria, Czech Republic, Slovakia, as well as the adjacent parts of Hungary and Poland, in a stretch of Central European land some 800 km (500 mi) across," according to Wikipedia. The most famous rondel is the Goseck circle in Germany:
"The [Goseck] circle represents the remains of the world's oldest observatory, dating back 7,000 years. Coupled with an etched disk recovered last year, the observatory suggests that Neolithic and Bronze Age people measured the heavens far earlier and more accurately than scientists had imagined." Source: Scientific American (2009?)[subscription required for full article].