Depart from Castellina and at the intersection of the main roads, SS 222 and SS 429, take the asphalted road towards San Donato, pedalling up hill along the trail of Macia Morta, which offers panoramic views of the Val d'Elsa and Poggibonsi. After about 3.5 km, the road begins to descend and a steep decline begins; at this point you need to look out for the left turn which will take you towards Olena (about 6 km from Castellina).
Continue along the gravel road, through the town of Olena, near a drinking fountain and then turn left again where you'll be confronted with a short but difficult bit of road. When you arrive at a large oak tree, turn left again until you get to a gravel road. Turn right here through an area of vineyards, Mediterranean brush and pine and beech and oak trees. After 3km, the asphalted road begins again and you'll continue towards the Castle of Paneretta and the town of Monsanto before arriving at the bottom of the valley crossing the Siena-Firenze highway. The descent ends here at the level of an old abandoned building. Turn left on a gravel road that, shortly after, forks. Stay to the right and get ready for a major ascent to arrive at the village of Somano and, after another kilometre, the asphalt road of SS 429.