|Greetings from the Critique Club!
Very nice atmospheric photo, I also love shooting in old barns, sheds and the like :-) Good tones, nice long exposure and tones. In terms of horizon, I find it best to do what you seem to have done here, and ensure that the horizon of the focal point (window) is level. Trying to get the rest level will drive you crazy. I feel the centredness of the comp is a weak point, having it offset to one side would make it much stronger.
Also, beware the pp trap. It's too easy to go overboard with pp, especially tonemapping, and I hate to look at a shot and see the pp before I see the photo. I would definitely do no more than what you've done here, unless you are going for a more cartoony image.
Overall a pleasant shot that finished well, so keep up the good work!
Feel free to PM me,