Head north from Arezzo along Regional Road SR71 until you reach the small town of Rassina. Just a few kilometres from here, you can visit the Church of Sant'Antonio in Socana, an extraordinary example of worship through the ages: first used the Etruscans (there is a large sacrificial altar in the apse, part of the Etruscan temple that dates to the 5th century BCE), the Romans then chose it as a site of worship, followed, finally, by the Christians.
Back on Regional Road SR71, continue to Bibbiena. Right before the town, take a left at the roundabout onto Regional Road SR70, also known as Strada della Consuma, which will take you to Poppi.
As you near Poppi, you'll see the medieval castle that belonged to the Guidi counts from a distance. Built in the 13th century, the castle's design is attributed to Lapo and Arnolfo di Cambio, with later changes in the 15th century by the local architect Jacopo Turriani. The large building is a gem of civic and military architecture, excellent evidence of the wealth that the Guidi counts acquired over the years. Now that you're here, make sure to stop by the counts' chapel at the castle, which boasts frescoes painted in the first half of the 14th century by Taddeo Gaddi, one of Giotto's most faithful followers.
The historic centre of Poppi is also worth a visit. Take a stroll along the main street via Cavour, lined with porches, and pay a visit to the religious buildings standing at both ends of the street: the Oratory of the Madonna del Morbo, an elegant example of Baroque architecture (17th century), and the Vallombrosan Abbey of San Fedele (late 12th century).