Villas & towns good as a base for ~15 people for a month (September-ish) for non-car area travel

What do you want to do with your month? My knee-jerk reactions have been places that are tourist lake resorts, but for an entire month, maybe you want to be an easy commute into a big city that you can then hit up at will?

If that's the case I would frame my thinking as "a villa outside of ...." And then insert some of your favorite cities where you might want to spend time. Potsdam area just for instance, which is a lovely good living place particularly in the summertime, might be a good choice if you were partial to the idea of popping into Berlin whenever you felt like. Much to be had there lakeside.

If I didn't care about getting into a bigger city easily, then I might pick places that connect fairly quickly with more than one high-speed rail line. That way you could still go buzzing off to many different places quickly. Maybe around an alpine resort town that quickly links to Innsbruck by rail. From there, you have easy high-speed rail connections to Italy, Switzerland, Germany and the rest of Austria. Again just for instance maybe Seefeld? Places that have ski lift networks have the advantage of many large rental units.

My compatriot did this last year with a rented villa on Sardinia's south coast. But they didn't travel much. I can't imagine getting around Provence without a car, or Tuscany either. I could see doing this near Porto, but it would have to be on the main bus or train line, both of which are cheap ways to get around provided you want to go where they lead.

Hank is correct in thinking you want to be near a transportation hub unless you just want to stay, more or less, in the local area.

Consider Stresa, Italy. Gorgeous setting on Lake Maggiore. On the train/bus line between Milan and Switzerland. Tons of gorgeous towns along the lake to explore. Easy day trips.

Regarding the no car and great mass transportation plus walking & cycling, you CAN'T BEAT the Netherlands. I don't immediately think "ruins" WRT the Netherlands, but they do have some of that. Add in countless museums, tons of cheese, markets, nightlife, cafes, and great food. Amsterdam is an easy option for radiating out to other small town and largish cities, and most are very easy to get between. We choose to mostly cycle to and from places (great for the younger folks and more fit older ones), but the trains and buses are pretty great too. Can easily add longer day trips to Belgium or Germany.

THANK you for all these wonderful suggestions! Keep them coming! Most of our group will be happy to bike, walk, bus or train places so all these ideas are helpful. And I agree, I am thinking a smaller town with great transportation so folks can make day trips exploring other towns/cities. If anyone has villa/hotel/ Airbnb recommendations I would also value those too!

Many great options.
You will need easy access to Rail. Not only finding the right city or town, but finding a place to rent for 15 persons that is near rail.

I have been all over Europe and will try to avoid picking my favorite places. Many places are good for this.

The rail systems in Eastern Europe are not as developed as in Western Europe. Also, I love Scandinavia, but would advise that it is the most expensive area in Europe, along with Switzerland.

Since you lived in Vienna, you probably would not wish to do this in Austria where you have lived.

Italy has a great rail system and some great choices. Florence has much in the way of art, but it might be a bit too large for you.

Consider Siena, Orvieto or Spoleto.

I love Porto and the Douro River Valley that has a nice train that runs along the north bank of the river. If Porto is too large for you, consider Regua or Pinhao. There is a wonderful BNB in Porto called the InPatio Guesthouse that is one of a kind. It is not a villa and only has 5 rooms, but perhaps you could rent the entire place. It does sell out early.

Great Britain is wonderful to visit, but you would need to pick a place with easy rail access. Consider a medium sized city, York. Also, Winchester.

The Black Forest in SW Germany is nice, check out Triberg or Friberg. If you want beaches, Croatia is nice, but not sure of the rail connections.

Budapest is wonderful, but you have probably done it.

There are luxury travel planning agencies that might be more suited to helping out. The volume of people in your group perhaps makes suggestions here a bit provincial rather than practical. I watched a family member get a managed holiday that was quite impressive. Treat the trip with the important planning it deserves.

How about in eastern Germany close to the Baltic ? Basically empty of foreign tourists, your group just might the only ones. I would suggest Greifswald and that vicinity., some place quite different in small towns.

I like the greater Potsdam area, lots of real estate designed and suitable for bike riding. You see locals doing this all the time. Potsdam an der Havel as well the other close town, Werder an der Havel...interesting architecture, the river cruise on the Havel, which I had an opportunity to take a second time thanks to the weather in June, could easily spend a week here in this Prussian " backwater " area.

Thanks once again for all the thoughful , insightful remarks. I will investigate them! Many thanks.

Another approach is to search for the house on AirBnB, using all the relevant filters, zooming out to all of Europe. Choose your favorites and then research each location.