Siena is a city a small city, yes, but it isn't like one of the hill towns and the attractions away from the Piazza/Duomo area are spread out on three steep hills, so walking is a necessity. You will understand why Italians can eat so much, and not get fat, when you see old women carrying groceries up a long street with a 30-degree incline. If you are tired, check to see if you can get to your destination by walking along a ridge, rather than going in a straight line down a hill and back up.
The centre of Siena is accessible only on foot. Cars other than taxis, police, etc. are strictly prohibited; motorcycles and scooters are OK, though. Patrons of the central hotels are allowed to drive up and unload the luggage and then get out, but only by obtaining one-time permission slips from the hotel front desk beforehand also have them draw the route for you on a map and follow it to the letter; if you miss a turn, it may be wiser to head out the nearest gate, get on the circumferential road just outside the walls, return to the starting point and try again; have this pass handy if stopped by police while driving within the walls - or, in a pinch, at least a confirmation of your reservation. Don't rush your turns, and swing wide like a truck, as you would be sometimes required to fit between two stone walls into an opening just slightly wider than your vehicle. Outside the main city walls can be found various parking areas. For more information, contact "Siena Parcheggi" tel. 0577-228711. To call or reserve a taxi, telephone the Central Reservation Office at 0577-49222.
Siena may be the only city in Mediterranean Europe where parking is not a massive headache, though charges have increased dramatically in the past few years and you can expect to pay €40,00 or more per day for the more convenient spots. The huge parking lots around the Fortezza and the adjacent football stadium are no longer free, but on the other hand, you can now count on finding a space there almost anytime; there is free parking further out, with minibus service, from Due Ponti and Coroncina beyond Porta Romana.
Google maps shows the location of all bus stops within the city. If you zoom in and click the bus symbol on the map, you will get a list of bus routes serving that stop. There are several small buses Pollicino run by the TRA-IN company that cover some streets located in the centre and several bus lines to and from the outskirts of town. Bus tickets cost 1,10 € per fare when bought at kiosks/tabacchi but are more expensive when bought from the driver.
The website for Siena Mobilità (http://sienamobilita.it/o...) has bus schedules orari for routes within Siena. Click the tab Servizio urbano Siena.