I am looking for a good chess website about retrograde analysis problems, but after a few searches, I haven't found anything. Are there any websites that you can recommend?
I would recommmend The Retrograde Analysis Corner It is a top notch and well known site that explains castling and en passant conventions, what exactly it is all about, and how to solve such problems among it's many, many helpful things.
Do not miss Rund um die Retroanalyse, a blog by Thomas brand, in German language. It features each week a retro problem, with explanations and the problem solution.