1056000 / 649 = 1627
Of course, this is just by square footage. It does not take into consideration the day rooms and corridors, or the volume of the interior walls. So you would probably reduce the figure by at least 25%.
Still, that's a lot of capacity. The castle was designed like a college campus, and these cell rooms were for single students.
Hmmmm...single "students," huh? (What shall I study, pray, tell?)
Are the walls thick enough that you can't hear the stereo from the people next door? I once lived in a dorm where the walls (ceilings, floors) were so thin, you could hear the people upstairs when they punched the buttons on their touch-tone phone.
"Oh, Amy's calling her boyfriend again! That means we'll have to listen to them bicker for the next two hours ..." Once I aimed my stereo speakers to the ceiling and turned on a Wagner tape at full volume.