From just in front of the castle parking area in Sillans is a walk that is well sign posted, through the olive groves, leading to the bottom of the large double waterfalls that Sillans La Cascade is famous for.
You will not be able to get too close to the waterfall but the river below it with its own turquoise mini-waterfalls are worthy of shaking away a few branches and spiderwebs to get there. Just follow the signs and be very careful (use good walking shoes). 30 mins walk from chateau and 30 mins back. Sillans La Cascade is about 12 km from Cotignac and a 10 mins drive away to the North, on the way to Aups.