So you're telling me, after all these years; none of the prisoners that have been in that cell happened to touch/push on that one chest-high brick... Opening the path to their freedom?
Also; when they went to leave and realized there was a prisoner in the cell: absolutely 0 questions about who they are, why they're in prison/what their crimes are... Just 'fuck it' let them tag along, effectively unsupervised, behind the group, collecting weapons, in the perfect position to ambush the Emperor?
The blades don't seem very good at their jobs... It's a wonder the Emperor made it as far as he did.