It had a central hall with four radiating wings, all visible to staff at the centre. This design, intended to keep prisoners isolated – the "separate system" first used at Eastern State Penitentiary in Philadelphia – was not, as is often thought, a panopticon. Officers had no view into individual cells from their central position. Pentonville was designed to hold 520 prisoners under the separate system, each having his own cell, long, wide and high with little windows on the outside walls and opening on to narrow landings in the galleries.