These "temples" that they refer to strike more of a note along the lines of military defences as opposed to structures used for religious purposes. What with the ramparts and the ditches. Also, it makes sense that if you are digging defence ditches and are under a limited time, the greater the diameter, the less time you have to dig deeper ditches.

 

This also reminds me a bit of the 4 great cities that the Tuatha De Dannan were a part of. Falias, Gorias, Murias, Finias. I'm not sure if it fits within the "accepted" region for these cities, but its a possibility isn't it? A naive theory, built upon my original impressions, could be that these cities were under attack and defensive measures took place in the form of retreat to Ireland. Most of the times in the past, when one culture wanted to move in a new territory, trade usually took place before hand with the currently residing culture. It is of interest to note that the Tuatha De Dannan did not hold trade with the Firbolg before their invasion (atleast not that we know of), so it could be they were under dire circumstances, thus the sudden invasion. I might not have read it, but it makes me wonder why the Tuatha De Dannan did not stay in their 4 glorious cities.

 

Tad  

Central Europe has faced invasion after invasion and I'm sure the people in this civilization were always prepared for warfare. Maybe that's what wiped them out so soon...