Okay, I got it to work thanks to everyone's help. Here's what I did: Start with the black Walls layer and: 1) Duplicate layer (Layer>Duplicate Layer) Note:You will be sitting on the duplicate layer ready to edit it. 1.5) I named the new layer "Shadow." 2) Filters>Blur>Motion Blur; 2.5) For motion blur settings I used length=10, angle=225; 3) Select>By Color and click in the background; 4) Select>Invert (this will result in all the blurred areas being selected) 5) Fill the selection with black; 6) Filters>Blur>Gaussian Blur; 6.5) I used a horizontal and vertical blur of 70 just messing around; 7) Drag the Shadow layer beneath the Walls layer. The map so far (WIP): Thanks for everyone's help.