15 Prisoners

Here’s a puzzle that’s been on my wall since I was eight. 15 prisoners:

Annoying optical illusion - 2

Rotate the upper surface and…

Annoying optical illusion - 2

14 prisoners. Where did he go? Click on the images for larger versions.

I hate this puzzle. Even knowing the answer, I simply cannot get an intuitive grasp on it. I’ll post the explanation tomorrow.