My wife and I would like to spend a week in Chianti. In other online forums, several people recommended Castellina. Is Castellina a good choice for my wife and me? I have a lung condition that leaves me short of breath when wallking uphill. My wife uses a cane and can't walk more than a few hundred meters. How steep are the streets in Castellina? Will parking restrictions in the center make it difficult for us? Is Radda any different in this regard?

The steepness of the streets is relative to each individual, so that's hard to answer. It's been some time since we've been to Castellina, but I don't remember a lot of steep inclines (again, relative). You might try looking at street view in Google. There's a parking lot just south of the ZTL. Would you be able to walk out of it?

Tuscany is pretty much hilly. I've been to a fair few places and only two are really level

https://en.wikipedia.org/wiki/Buonconvento is on the valley bottom and has much of its walls left, a street market and some nice, simple restaurants.
Pienza https://en.wikipedia.org/wiki/Pienza is on a ridge and, as long as you stay in town is pretty flat.

North of Siena there is a small valley which has little of interest in it that is not hilly

Lucca is dead flat but harder to drive around

Pisa is also dead flat but see Lucca

I also don’t remember walking up hills and recall it as being fairly flat.We enjoyed staying there with friends.

castellina

You might also type the names of cities/villages you wish to visit in the search box on YouTube. There are several videos of specific villages shot by amateurs and professionals walking about the towns.

Thanks to everyone for your responses. I see that the streets are not completely flat, but most are not too bad. I think the best solution for us would be to stay at the Hotel Palazzo Squarcialupi, which is actually in the town, where we would have restaurants within walking distance. A little late to get one of the best rooms this year, but we'll plan on booking early next year.