With reference to post 4 by alvinylaya: The Sun/Moon Shadows intensity setting won't make any difference to the illumination of the unlit side of objects (such as the problematic castle); only to the lightness or darkness of the shadows those objects cast onto other objects (such as the terrain). One of the keys to better realism with simple render techniques is achieving a similar balance of the brightness of unlit areas of objects (ambient material setting) and areas under shadow (Sun/Moon Shadows intensity). That's pretty much a case of doing it by eye, since materials will respond differently to the ambient setting.