Is Orvieto a good base when you rely on public transport to visit few medieval towns

Knowing whether you have a car or not would help better respond to your question.
In any case, car or not, those locations are too far to be properly visited from one location only.
Orvieto is easily reachable from Rome in about 75 min by train, so it is an easy day trip from Rome.
Civita is a bit more challenging. From Orvieto it is an easy 50 min bus ride, but from Rome it requires longer so it would be more difficult to accomplish in a day.
Florence is 90 min from Rome via bullet train but that train doesn’t stop in Orvieto. Florence also has too much to see to be visited on a day trip,from Rome, albeit it is doable.
Siena is almost 3 hour from Rome by bus, or 75 min from Florence.
If you base yourself in Siena, then San Gimignano is an easy day trip. Volterra is doable also as a day trip from Siena. To do both San Gimignano and Volterra on the same day from Siena would be challenging, unless you have a car.

Hello zinmaking, and welcome to the forum!

There is a bus from Orvieto to the modern town of Bagnoregio which has a longish (2k) walk to Civita di Bagnoregio and occasionally there is a shuttle. Check the forum for recent discussions about this bus which doesn't run on Sundays.

Siena is worth a visit and is a better choice as a base for San Gimignano since it is closer and a hub for transportation since it is the largest town in the area. (Even just driving from Orvieto to San Gimignano is probably 2+ hours one way.) Using public transportation for San Gimignano from Siena involves either a train and a bus or two buses transferring in Poggibonsi. There is a train station in Poggibonsi that has been rebranded Poggibonsi/San Gimignano but understand that there is still a 25 min bus connection to SG from there it is not in the town itself.

There is a bus that runs from San Gimignano to Volterra but depending on the time and the day your choices will be limited - specially trying to get there and back to SG to get back to Siena. This is problem with public transportation in rural Tuscany where it is possible to get from one small town to another but for the tourist getting there and then back becomes a real problem where your day is dictated by the only scheduled bus that runs a couple of times a day. If you really want to see both in one day from someplace like Siena I would look into a tour or planned excursion and have someone drive you.

I find that while train information is easy to get bus information seems much more difficult to pin down from outside the country without an understanding of how the system works. Also I was unable to lock down the location of Florenz - can you clarify where this is?

Rural Tuscany is a challenge by public transportation so pre-planning is important, but it is as beautiful as advertised so it can be worth it.

=Tod

Thanks Tod for an elaborate response.
With Florenz I meant Florence. Edited the post now.

I do not intend to take car. I am visiting for 16days in total.

I am planning to base in Rome for first 8 days.
5 days in Rome.
Day 6 - day trip to Pompei.
Day 7 - day trip to Orvieto from Rome. Train takes ~1hr from Rome and is also cost effective.
Day 8 - day trip to Civita di Bagnoregio from Rome. Train to Orvieto and then bus to Civita di Bagnoregio (I dont mind the long walk).

Next I have 3 days before I go to Milan. So I think to shift to Florence as a base for the next 3 days as follows.

Day 9 - shift my base to Florence and go around the rest of the day.
Day 10 - explore more of Florence .
Day 11 - Day trip to Siena or (Volterra+San Gimignano) .
I am not very sure if Volterra+San Gimignano is feasible in one day.

I wish I can do all 3 Siena+Volterra+San Gimignano. But I am unable to accomodate all 3.

For last 4 days I am shifting base to Milan (From where I have my return flight tickets)
Below is my plan for last 5 days.
Day 12 - Shift to Milan and go around Milan.
Day 13 - Day trip to Lake Como.
Day 14 - Day trip to Venice.
Day 15 - Explore Milan.
Day 16 - fly back home

Leave Rome on Day 7
Stay in Orvieto for 2 nights before Florence rather than all that back and forth

Day trip to Venice from Milan is a bit too far
Usually a 2.5 hour fast train x2 that is 5 hours spent traveling

It is an ambitious plan. These are my 2 cents.
Since you plan 3 day trips from Rome (Pompeii, Orvieto, Civita), then you need at least 6 nights in Rome. However you might consider to shorten your stay in Rome and have a short stay in Orvieto to visit both Orvieto and Civita while there to avoid multiple day trips to the same area.
Florence is a good base for Tuscany. If you plan 2 day trips from Florence then I suggest 5 nights in Florence. You need 2 full days for Florence alone (3 nights). With 5 nights you can add 2 day trips. Although rushed, you can couple Siena with San Gimignano on a day trip. Start with Siena by bus from Florence, then bus to SG, then bus SG to Poggibonsi. From Poggibonsi you can take the train or the bus back to Florence.
Volterra is more cumbersome to pair with San Gimignano without a car, so it is best to visit separately from Florence. Take a train from Florence to Pontedera (on the Florence-Pisa line). From Pontedera there are numerous buses to Volterra. To get back to Florence you can do the reverse.
At this point you have 5 nights left (I’m assuming a total 16 nights on the ground).
I would spend 3 in Venice. Then move to Milan for the last 2 nights. You can see some of Milan on the day you arrive from Venice (maybe half day). If you stay 2 nights you have another full day before your last night. You can take a day trip to Varenna (Lake Como) which is less than 75 min by train, assuming the weather is good (October days have a chance of rain), so if it is a rainy day, skip the lake and visit more of Milan.

Not surprisingly Roberto's plan is very good and I agree with him on all his points.

I think the quiet of Orvieto will be a great change after the chaos of Rome and it makes much more sense to move there and experience Orvieto in the evening and cut out all the repeated day trip travel. Orvieto is on the path of Rome to Florence so it is really just a stop on the way of a larger move.

Venice is really worth the time spent there and it deserves to be much more than a day trip.

Have a great trip,
=Tod