Durham Castle is set just in front of the Cathedral, there is a gatehouse and large wooden doors at the far end of the green. The tours are booked I believe, we were exceptionally fortunate as it is also student accommodation and one of the brood was going to accommodation there so we dined in the great hall (see photo) and had run of the place. The black staircase is amazing, you feel seasick walking down it as it twists and turns as you descend. There are displays throughout the castle. As it is also student housing tours are limited and you generally don't get to see everything there is to see. At £5 per adult its a must.