Tuscany: Staying Inside City Walls

Try Lucca.

Lucca, Siena, Florence, and pretty much every ancient town has city walls.

To Roberto De Firenze:

I meant where the walls completely enclose the city.

Thanks.

Lucca has extensive city walls with a park and bike paths on top of them.

https://it.m.wikipedia.org/wiki/Città_murate_italiane

Of the provincial capital cities, the only two with the walls totally intact all around are Lucca and Grosseto.

Florence has intact walls only on the Oltrarno side of the river. Siena, Pisa, Pistoia, Arezzo, have walls but a few gaps.

Of the small fortified villages, Monteriggioni is probably the most famous in Tuscany, although Cortona, Lucignano, Anghiari are also some of the most notable.

Lucca is one of my favorites.

I'm not personally recommending it, but you might consider Monteriggione-based on your description of what you want. You might have to settle for a B&B there, and it can be touristy in the daytime. But if you have a car, it's a great base for touring. I personally would prefer a larger town, with a wider variety of evening walking and dining, and a few monuments of art or architecture.

I would point out that many, many towns have substantial parts of their walls left, although the entire town is not encircled like Monteriggione. And just being on a hilltop, like many towns, emphasizes the fortress-like feeling. I'm not urging you to compromise, only to take a more nuanced look at what will make your location a highlight of your trip. I also like to have air conditioning and a pool, but that may not be important to you. (I'm not a "pool" guy, but the heat in Tuscany changed my mind.) Month of the year and budget make a big difference.

If you opt for Monteriggioni, which is small but I definitely recommend if you plan to rent a car, there is actually a very nice hotel inside the walls (linked below). It’s not cheap, so if you prefer something cheaper there are a couple of B&B also.

Monteriggioni was a town mentioned in Dante’s Inferno, so you can say you went inside a town that was mentioned in a 13th century masterwork.

http://hotelmonteriggioni.net/it/?language=en

The problem with Lucca is that it's on a flat plain, so no views from the walls.

Orvieto is just barely on the Umbria side of the border with Tuscany, but is on a hilltop secured by massive fortifications, in case that fits your bill. Take a look at some photos and videoclips I made there to see what you think:
https://flic.kr/s/aHsjvcWE1Q

Lots of lodging inside those walls.