Introduction:
The first stone defences in Plymouth were those of the 15th century Plymouth Castle of which only a very small part of one tower remains near the Citadel. Before this the main local defences were at Plympton, but the importance of this castle faded as the River Plym silted up.
In the first half of the 16th century six blockhouses were built along the shore. A fort was constructed in the 1580's on Drake's Island and in 1596 Plymouth Fort was built on the Hoe.
After the Civil War the most visible parts of the present defences were added, the Royal Citadel between 1665 and 1670 at the south of the city on the upper part of the site of Plymouth Fort and Mount Batten Tower on the opposite shore in the 1650's.
In the middle of the 18th century fortifications were built around Devonport dockyard. A number of earthwork batteries were also added around the northern end of The Sound. The defences were expanded at the end of the 18th century following concerns over possible French & Spanish invasions.
In the second half of the 19th century new defences were planned by Palmerston consisting of six coastal batteries and a ring of eighteen land forts and batteries. The forts were polygonal in design; each side was covered by fire from galleries, known as caponiers, protruding into a surrounding ditch. Guns mounted on the flanks of the fort were vulnerable to fire from the side so were placed in bomb-proof chambers or ‘casemates’.
During the second world war, Plymouth defends itself from the aerial bombardment using 3 ½ inch QF Heavy Anti-Aircraft weapons mounted in some cases to existing fortifications. In 1943 the air defences saw the deployment of the UP rocket projector battery. Unrotated Projectile, or UP, known as Z Batteries were a short range anti-aircraft weapon which launched 3-inch diameter rockets, used in ground-based single and multiple launcher configurations.
The Rocket
The rocket unit was approximately 6 feet long and 4 inches in diameter. At the business end was an 18 pound high explosive shell equipped with a brass fuse that was something special. In place of a pointed tip to the fuse was a flat surface about one inch in diameter as if the tip had been sawn off. In this flat was a series of small holes drilled parallel to the longitudinal axis of the rocket. Airflow produced by the accelerating rush of the rocket skywards built up a pressure on an internal "top-hat" shaped piece that at a pre-set time started the initiation of the explosive in the shell. Within the main body of the steel rocket casing was a propellant of solid Cordite. At the extreme tailend were four small steel clip on fins arranged 90 degrees apart. Between the fins were electrical contact pads to allow contact with the fixed knife-blade contacts on the projector.
The Projector
This was simple vertical steel structure standing on a circular metal base, mounted close to the ground that must have been at least 6 feet in diameter. It was marked clearly in 0 to 360 degrees azimuth. Two pairs of long rocket guide rails (rods) were arranged parallel to each other and in the same plane. They were pivoted about the centre of their length to permit elevation of the rounds.
Mounted to either side of the vertical main frame holding the tiltable launch rails was a horizontal ammo rack at knee height with built-in fuse setting key. On each side of the main frame stood the gunners No1 and No2, these being the gun crew.
No1 on the left hand side (when facing the rear of the projector) was responsible for the setting of the fuse to his round, setting the projector bearing (by pushing on a horizontal bar at waist height) and the final firing of the weapon. For receiving orders during action No1 would permanently wear earphones normally stowed in a tin box at hos position. Gunner No2 on the right hand side of the projector was responsible for fetching two rounds from the ammo shed, placing them in the No1 and No2 racks, setting the fuse on his own round, operating the elevation by means of a crankwheel and finally holding in a button to make partial electrical connection prior to No1 closing the circuit through the final firing lever
The Gun Laying (GL) Radar, code-named 'Z4', was based at Ridge Cross at Staddon Heights, Plymstock, for two years from 1943 to 1945 and detected approaching German aircraft. A memorial stone is located at 50° 20.683'N 4° 4.579'W.
The cache is located within 100 metres of the original location of the battery.