From Torbole, head towards the historic center and then turn left towards the Parco delle Busatte (sign). Follow the narrow road to a hairpin bend where you can already see the grey slabs. Park there.
Continue briefly on the paved road until you reach the Sentiero dei Paloni on the right (signpost). Follow it and out of the woods and enter the area of the Marocche; after a few meters you turn right along a track in the woods, with not very visible orange and blue marks, which soon lead to the base of the wall.