I'm going to steal this one as soon as my time machine is complete. It vanished in 1945 and is presumed to have been destroyed.
I love the tall ruin, graceful even in decay, and the way it's framed by the crippled trees and the eerie light. No-one paints melancholy and loneliness as beautifully as Friedrich. His landscapes always feel to me like they are totally silent, with no human around for miles.
A few years ago, the Kunsthalle in Hamburg hosted a huge Caspar David Friedrich exhibition and I got the chance to see many of his works. What fascinated me most were works he did for Nicholas I on transparent paper. The painting were illuminated from behind by daylight or candlelight shining through a glass bowl filled with water or white wine, a so-called Schusterkugel (shoemaker's bowl). There's only one if these paintings left, Gebirgige Flußlandschaft, but the Kunsthalle showed three drafts Friedrich did as well and even had music composed since the music that was meant to be played when the painting were exhibited was lost. If you ever have the chance of seeing this, I really recommend it, it was amazing.