From Pisa Airport:
Get to the central train station, Pisa Centrale, through the shuttle train called PisaMover, and from there get a ticket to Siena. This transfer requires changing trains in Empoli. It is possible to buy a cumulative ticket for both the PisaMover and the train.
From Florence Airport:
Catch a tram to the central train station, Santa Maria Novella, and from there either get a train to Siena (may require changing trains in Empoli) or reach the nearby bus station to get bus 131R (recommended option).
From Rome Fiumicino Airport:
There are several available options. It is possible to catch a relatively fast bus by TiemmeSPA from the airport directly to Siena, although it has only two runs per day. It is possible to catch a FlixBus from the airport to Siena, although it takes a bit more time.
Alternatively one can arrive to the central train station, Roma Termini, using the “Leonardo Express” train, and from there get a train to Siena. This transfer requires changing trains either in Grosseto or Chiusi-Chianciano Terme. Alternatively, reach Roma Tiburtina station using the regional train FL1, and from there get a direct Flixbus.
From Rome Ciampino Airport:
Once per day direct bus connection to Siena is available. Otherwise, one can use local buses to Rome train stations, and from there continue by train or bus to Siena.
From Perugia Airport:
From Bologna Airport:
Take the train shuttle service “Marconi Express” to Bologna centrale. High-speed trains connect Bologna to Florence Central Station “Santa Maria Novella”, from which one can proceed as described above. Furthermore, there is a bus from Bologna Airport to Florence Central Station. Finally, there are also buses from nearby the station to Siena.
Once in Siena…
If you came by train, you can get escalators in the Galleria Porta Siena shopping mall, in front of the train station, uphill to Antiporto. From there you can walk until Porta Camollia to get in the city walls.
If you came by bus, the recommended bus stop to get to the city centre is Piazza Gramsci (or via Tozzi).
If you come by car, free parking spots are available along the city walls around the fortress (here and here), or close by. There are also parking spots close to the conference venue, in San Francesco, but they are not free. You can then reach the Complesso Universitario San Francesco with escalators.