Sounds yes like the old abbey or monastery or whatever on the north side of the Mosel opposite downtown Trier - a lovely walk up thru the woods or I believe their is a Sesselbahn (sp?) from the south bank in Trier to it. It is really in a sweet sylvan setting if that is the right one - I stayed off and on in Trier for years and being an inveterate jogger used to jog up that way often - nice walking/biking paths along the Mosel too though this stretch of the Mosel is not that scenic all in all.

It's possibly the Felsenkirche in Idar-Oberstein, on the Nahe River, not all that far from Trier.

http://upload.wikimedia.org/wikipedia/commons/1/14/Idar-Oberstein_Felsenkirche.jpg

Idar Oberstein occupies both sides of the river valley at that site.

Might also be the Marienburg on the Mosel - not really a cliffside, but definitely a complex of buildings.

http://upload.wikimedia.org/wikipedia/commons/2/20/Mosel_Marienburg_Umgebung.jpg

http://www.zell-mosel.com/html/impressum/impressum.html

The Marienburg overlooks several towns:

http://www.ferienwohnung-mosel.com/images/Schifffahrt.jpg

Thank you both for your help. I realized I asked a very good question, because even though I don't think any of your guesses to my very vague question is the one I had seen, they are all interesting places for me to consider visiting as well!

Felsenkirche in Idar-Oberstein looks bizarre, the sort of place that keeps making me want more time on my trip!

I have some time before my trip and hope I can stumble again upon what I had seen. I'm not sure it is very near Trier, but it is on the Mosel. The buildings, including a medieval chapel or small church, were still intact. The boat tour this traveler was on seemed to emphasize the outstanding views of the river from the terrace of the buildings.

Do you have a link to those photos so we can have a look at them?

That is my problem, I did not bookmark the page. I'm still looking for it, but at the moment I'm distracted by the other end of my trip - trying to figure out what to do with a couple of days between Rothenburg and Strasbourg.

Are you sure it was an abbey, not a castle?

http://davidpratt.zenfolio.com/p592206013/h388E13F#h388e13f

???

The narrative written by the traveler mentioned that it was an old religious community (probably not an abbey since google is not producing anything with that term) that brewed beer and sold it to the town across the river. I don't think it was a castle at any time, but I only saw the one trip report.

Thanks StCirq, that's not it. This was a simple medieval structure - about that size though - made with local unadorned stone.

Can't wait to find it to see how close my memory matches reality. It's probably somewhere in Italy! (just kidding)

Klause Kastel.

Whew! Thanks hetismij2, for making me search for castles!

It's not a castle, as far as I can tell, btw.

http://www.trier-info.de/klause-bei-kastel-info

Impressive! I had never heard of that building but looked it up now.

No, it is not a castle but a hermitage. Kastel is the name of the village, actually Kastel-Staadt ("false friends" lost in translation). The building in the photos is the burial chapel for King Johann of Bohemia, designed by Friedrich Schinkel in 1835 on the ruins of the medieval hermitage.