For anyone who is unknowledgeable about the ancient Celtic peoples, the oppida (singular: oppidum) were towns developed by the widespread La Tène culture of ancient Europe, contiguous with Classical and Hellenistic-era Greece as well as Republican-era Rome. They built these elevated, brick-walled settlements throughout a territory spanning what is now modern-day Portugal, Spain, France, England, Belgium, the Netherlands, Switzerland, Czech Republic, Slovakia, Hungary, and Serbia. They were the chief sort of settlements confronted and besieged by Julius Caesar in his conquest of Gaul. I've been able to find a number of modern-day reconstructions online, but very few pictures of the ruins and remains of these settlements. Of course none of the original timber architecture would have survived the ages, but I've seen very few actual stone facings left by the Celtic peoples along the ridge lines of the acropolis-style elevated terrain that formed the central locus of such settlements. For instance, among the many "castro" settlements in Galicia, Spain and northern Portugal is the oppidum of San Cibrao de Las, which has monumental gates and fortifications that have survived in ruins.
Do sites like this rank as the best preserved of the oppida, though? Naturally, over time, the Romans and then later Medieval peoples would have quarried such sites for stones to build other buildings nearby. Yet are there some oppida that are left largely untouched due to being remote and in the country, far from any later city or town? The oppidum settlement in the picture above looks naturally weathered due to the elements. Perhaps there is nothing better preserved than a ruin like this as far as ancient Celtic architecture is concerned, but I'm just not sure. I'd like to know if someone knows more!
Just for fun, here's a little reconstructed model of an entrance to a Gallic oppidum.