Obstacles are used to obtain a definite control, both as regards direction and speed, over the progress of troops advancing to the attack. Their chief value lies in their power to deflect the attacking troops into areas most favourable for their destruction by the defences. With this objective in mind they should be sited to:
Break up the unity of action and cohesion of the attacking troops
To deflect parties thus isolated into the best swept fields of fire
To arrest them under the close fire of the defences.
For obstacles to be under the close fire of the defences, they were should not be sited more than 100 yards from the parapet.
Two types of wire obstacle were constructed in Suffolk, high wire and apron. High wire entanglements were an effective obstacle; they should be as wide as possible and if possible constructed in two zones with a small gap between the two zones. Posts should be as stout as possible, to allow them to be knocked in by hand; digging them in made the obstacle less effective. They were constructed by joining each post to all the adjoining posts with taut wire head to foot and foot to head. Barbed wire was then hung in festoons between the posts, but on no fixed pattern.
Above: Wire obstacles (middle image shows an Apron Fence being constructed while right image shows a Hire Wire Entanglement)
These were used to close streets, roads and bridges. They could be constructed of any material at hand. Barricades were either moveable or fixed. If the obstacle was to be fired over, a bullet proof stockade could be constructed. Such a stockade was constructed to block off Felixstowe pier.
This obstacle was formed of limbs of trees firmly picketed down with branches pointed and turned towards the enemy. Strands of wire could also be interlaced to improve the efficiency of the obstacle. These were certainly constructed in the Felixstowe defences, using the material from hedge rows demolished to provide a clear line of fire.
Land mines could have great material and moral effect against attackers. Some mines were planned as part of the Felixstowe positions, to be fired electrically and supervised by the Submarine Mining Engineers based at Landguard Fort.