Making a Complex Photo Mask with a Pen Tablet

I recently purchased a Wacom Bamboo pen tablet to use in retouching photographs and doing photo manipulation. The tablet would make these chores much faster and easier than using my laptop's touchpad, which is what I've used for years. I had my chance to really test my new tool when I had to create a complex mask for a photograph I made at the Gold King Mine and Ghost Town in Jerome, Arizona.

I made various adjustments in my RAW viewer, and in Photoshop, to have the foreground look like it does below. But I wanted to darken the view out of the window so you could more easily see what's outside while keeping the foreground properly saturated.