Your sun is almost parallel with the camera view - of course it won’t give interesting shadow play as all the shadows will be behind the objects that casts them. Try to be more creative with sun angle. Run interactive rendering and move sun’s gizmo untill you’ll find interesting angles. Also it could be good idea to use AO (better yet combination of several AO maps) on the wood material to simulate weathering effects and accumulated dirt. This will makes edges to stand out even more.