Bancroft's castle -  Bancroft's castle was built by General William Bancroft in 1906 atop Gibbet Hill in Groton Massachusetts. Since 2000 the castle and trails became part of Groton's protected open space and was opened to the public. Given a choice of categories company, educational institution, artist, athlete, office holder, mean of transportation, building, natural place, village, animal, plant, album, film or written work, the text refers to which one?
Answer:
Building