A portcullis is an iron gate that moves up and down to stop intruders entering the castle. It is also used for defence when an opposition attacks the castle. There is a portcullis after the gatehouse and another one after the drawbridge. The one after the drawbridge is just in case people make it past the drawbridge. The other one is to trap the attackers in the gatehouse so some of the knights or guards can pour some stuff through the murder hole.
We got this picture from www.castlexplorer.co.uk
