The fort followed a basic twin towers design, the towers being of a simple drum design, 24 feet across and 1 foot thick. Inside these drums floors were installed to make areas for living quarters, workshops and stores. The deck over these towers was made of reinforced concrete, topped with a deck house.
Fort Knock John was armed with two 3.7” heavy AA guns as well as two light 40mm AA Bofors guns and 4 Lewis guns. Armament changed throughout the war; the Lewis guns were removed later in the war to be replaced by twin Browning .303s. It was anticipated that the forts themselves would present such a small target that a large expenditure of enemy bombs would be need to ensure a hit to put the fort out of commission, with no real gain to be achieved. The defences on Knock John and the other naval forts were therefore considered to be ample, and could have the added bonus of harassing passing enemy aircraft and shipping.
Up to 120 men could be stationed on the fort and all the necessary rooms for living and working on a military fort had to be contained within the relatively small area within the superstructure itself. It was not a particularly popular posting; men were stationed on the fort for 6 weeks at a time followed by 10 days off and 3 days at shore base. Crew members took up hobbies from painting to knitting in order to stave off the condition known as “Fort Madness”, a mental health condition only recognised by the medical profession in the last years of the war.
All the Naval Forts, including Knock John saw action during the war, but they had all been abandoned by the late 1940s. The guns remained in situ and the doors between rooms were all welded shut. The age of pirate radio stations during the 1960s brought the forts back into use, albeit illegal, Knock John tower hosted Radio Essex, whilst Sunk Head Fort hosted Tower Radio & TV.
The guns on Fort Knock John were finally removed in 1992 but the remaining superstructure in still firmly grounded on the seabed in the Thames Estuary. It remains in remarkably good condition, especially as it has not received any maintenance work for about 70 years!