I do find it amazing that even in a place like Germany with excellent state-level surveying and archaeology services, as well as at least two centuries of enthusiastic amateurs, there are still undiscovered castles lurking in the forests...
Here is a description of the site together with a photo: https://www.unsere-burgen.de/de/Burgen-im-LK-Reutlingen/Burg... Judging from the photo, there is no immediately obvious indication that this free standing rock was the site of a castle.
As with all anthropology/archaeology, there is a big difference between “discovered” [by science] and “known” [by the locals]. The locals most certainly know of every castle and even those castles that are now ruins probably have local legends/stories of how there used to be a castle in such and such place.
But none of that counts until an archaeologist/anthropologist comes and writes it down. They then bombastically proclaim to have discovered a thing! (that all the locals already knew about for decades/centuries)
Sauce: my partner studied anthropology and this was a running joke in her program. It is very rare for an academic to be the first to discover anything. Usually they get tipped off by someone local who doesn’t count
What we do have is knowledge encloded in place names ("Burgstall"), but locals won't be any wiser about the history of the thing unless they happen to be the type of person who collects and consumes books written by local history science buffs of earlier generations.
A chance discovery like this usually happens when some known (usually to science far more than to locals!) but not recently disturbed site has to make room for an infrastructure project and the archeologists are sent in to make use of the last chance to see.
My comment was about completely undiscovered ruins/sites. Those are rare. Someone somewhere (could be a notebook) usually knows about it.
As for “no oral traditions in a country with long-standing books” – the Brothers Grimm famously started writing down oral traditions in the 1800’s. That’s only 200 years ago :) For a long long time in most of europe nobody bothered to write down what the peasants have to say.
The concept we both seem to be tiptoeing around is the idea that the best conservation approach is leaving it in the ground, for future archeologists to explore or not explore. I believe that this is a central principle of the modern approach to archeology, and one we laymen struggle to really take in. Yes, there are many sites known to science, but not analyzed to destruction by science.
In the case at issue here, the rock was called "Burgstein" and the forest around it "Burgholz". This names were already mentioned in a book from the 15th century and a book from the late 16th mentions a castle at the "Burgstein".[1] All this was long known,[2] but not all experts were convinced, that there had indeed been a castle on the rock. The new excavation now makes it seem even more likely.
[1] https://www.unsere-burgen.de/de/Burgen-im-LK-Reutlingen/Burg...
[2] For example, one Gottlieb Fri[e]drich Rösler wrote in his "Beiträge zur Naturgeschichte des Herzogthums Wirtemberg" (2. Heft, publ. 1790): "Auf dem Burgstein einem grossen Felsen gegen Abend, auf dem die Aussicht nocht schöner ist, sind auch die Spuren der alten Burg unkenntlich geworden ..." ("On the Burgstein, a large rock towards the Evening [the West], where the view is still more beautiful, even the traces of the old castle have become unrecognisable ..."). See p. 81 of: https://books.google.de/books?id=BY05AAAAcAAJ&printsec=front...
See also "Your finger, you fool".
RIP, Terry Pratchett.
This archeological site is a pile of rocks that once was a castle. (The rocks are probably mixed with dirt and plant life and might have been completely buried.)
But I do recommend visiting Germany, where there are way, way more castles than you might expect. In particular I can recommend the "Castle Way" along the Neckar River. Every bend in the river has its own castle, just dozens and dozens of them.
Most are not especially dramatic. Many are just ruins; others are literally just people's homes now. Still, it's a little bit like a D&D fantasy world where there really are just castles everywhere. (And yeah, some of them have genuine dungeons.)
(1) https://www.dw.com/en/does-germany-really-have-25000-castles...
(2) https://www.scrapehero.com/location-reports/McDonalds-USA/
The purpose of those castles was to extract money from the river traffic.
It's no different than any "protection racket" or police force: the money has to come from somewhere to offer protection, so some form of tax is normally used.
There is an immense number of dilapidated castles, keeps, strongholds or oppida all over Europe, with most of those barely visible to an untrained eye, and they have been catalogized for decades.