There's a hatch in the bottom of the fuselage, too.
"When installed in the turret either via a step into the turret's rear doors, or through the hatch in the bottom of the fuse lage, the gunner would plug himself in, so to speak, to enable his own supply of oxygen and radio communication with the pilot."
"Due to the confined space of the turret, the gunner was not able to wear the conventional parachute as worn by other crews. Space was available in the rear of the fuselage to store the gunner's parachute but there was little possibility that the gunner could leave the turret, put on the parachute and still manage to exit with this through the escape hatch provided in the floor of the fuselage. To alleviate some of these problems, a parasuit was designed for the Defiant gunners. This could be worn by the gunner while in the turret and he could escape via the turret doors. The suit itself could be inflated if having to bale out over water.
As well as the dying gunner having to train and switch the guns if the position was reversed, the dying pilot had to lower the undercarriage before expiring! This was because the rear VHF aerial was in the belly and retracted automatically when the wheels were lowered. If still extended it speared the gunner if he exited by the bottom hatch!
Consequently, the fuselage escape hatch tended to be used mainly by the ground crews when servicing the turret while the aircraft was back at base."
(From Mark Ansell's "Boulton Paul Defiant - Technical details and history of the famous British fighter" ISBN 83-89450-19-4)
"Gunners also had standing instructions to have the guns to the rear when landing at night in order to deal with possible German intruders in the Circuit, However, very few did so because in those conditions both gunner's exits were blocked."
(Colin Bryant)
"... the guns were automatically cut out when spanning the tailplane or prop. If the gunner was killed it was supposed to be his last despairing action to turn the guns forward and elevate them 19 degrees. This cleared the prop and he could transfer the firing me chanism to the pilot by a switch. No one ever told me, however, how I was expected to aim them."
(Colin Bryant)
Mike