I have to say "the castle". Having had the chance of visiting quite a few, they always fascinate me and still do to this day. Not only did they have a military vocation, but they were places where common, everyday people lived and breathed (some would say breed, which is also true).
Their construction fascinate me: their location, the materials used, the time it took, the skills required, the engineering requirements, and the ingenuity required fascinates to me. Definitely my favorite.