If the outer guardroom were to be overrun the platform behind that wall would be manned.
Without the electric light the enemy would only see the pool of daylight (or lamplight at night).
The observer on the platform would have lowered the vertical door to block the passageway, and would have his boiling oil and molten lead to hand.
The enemy would not be able to pass the pool of light without casting a shadow that the defender would observe.
The enemy would desend the stairs, reach the closed door, turn, and find themselves face to face with a shoot full of boiling oil etc.