We were there in August last year and the main beaches were quite busy. There is a small beach at the back of the castle, down a few steps that was quiet every time we went. there are some rocks that go out into the water and its nice to sit out there and chill. The castle itself is well worth the climb, some nice bars and cafes and the views are superb. Hope you enjoy Tossa, we cant wait for September to go back there.