If you choose the poly plane you can go to Window/Relationship Editors/Light Linking/Object Centric and unlink lights from your poly plane, they will no longer affect the background. To do this simply click on the lights you don't want to affect the plane. I would approach it one of two ways, either unlink all lights from the ply plane, create a new one just for the plane and unlink it from your castle.....or create an imageplane from your camera and use your background picture there under Environment in the Attribute Editor. The imageplane will not be affected by those lights...you adjust the brightness right in the imageplane's attributes.
In the Light Linking window, lights affecting an object are grey-lined, lights that don't affect an object are clear. Once you click on an object or light you make the change in your scene, you don't need to hit enter
Red bellows of flame have blackened my stones
Convulsing my frame and cracking my bones
Hell's dragons of steel who roar in their chains
Crawl into my caves to suck out my veins.....
-The Mountain P.F.M.