Hinge it on the top for the same reason that you'd drop it down slots from above: Yank a cord, and down it comes, fast. And if somebody happens to be in the way, they get smushed. The portcullis is generally not meant to be closed except when the outer gate has been breached or is in danger of being breached.

As for books… I can't honestly remember much about the ones I read back in high school. In terms of construction and economy, Castles and Ruins by Iron Crown Enterprises is quite good and well researched, but I don't recall how much space it gave to design.