I started with the original color shot. Selected the sky and clouds. I used the levels tool to deepen the sky, then reversed the selection and edited the castle itself with the curves tool. Then I channel mixed to monochrome using an RGB combination of 65, 10, 25.

I don't like the region between castle and sky. It feels cut out. Or even blue screened if you will. I guess I need to perhaps experiment with feathering the selections to soften this border region. I haven't done much work with selections. Feeling inspired to tackle this very useful tool. I welcome any comments and suggestions.

I'm using "The Gimp 2.2".