Inside the tail, you can see the control rods for the aerilons and rudder. For a combat aircraft, there is very little armor to facilitate high speed. Behind the tail section are two of the four 20-mm cannon the Mark 22 carried. The original Spitfire carried eight .303 cannon, outclassed by the Bf109E's two 20mm and two .30 caliber machine guns. 20mm cannon shells explode on impact for effect, while machine gun bullets are just hot metal that makes a small hole. So the 20mm cannon are more potent than just being a bigger hunk of metal.
RJ Mitchell. A life in Aviation. Spitfire and Battle of Britain. RJ Mitchell managed Supermarine which built the Battle of Britain Spitfire, flying boats and Schneider Trophy winning aircraft. RJ Mitchell is remembered for the Spitfire and Schneider Trophy.