In 1927 the goverment launched a new plan which included the construction of several motorways between the main cities. Unfortunately several of these roads crossed the New Water Line, like the motorway between Utrecht and Den Bosch. This crossed the line near the Diefdijk, a 17th century dike which was an integral part of the Water Line. To defend the new road two new casemates were built here in 1935. The southern casemate was armed with an anti-tank gun, the northern had a MG that flanked both the road and the other casemate.

The casemates of Diefdijk were built according the W21-28 standards. This means that they could resist a bombardment with 21 cm guns and some hits of 28 cm guns. The front was 180 cm thick, the roof 150 cm and the other walls had a thickness of 100 cm. This is quite heavy for Dutch standards, but this was possible because the casemates were paid from the larger budget of the Ministry of Waterways and Public Works. This also paid the new roads. Diefdijk Zuid was originally armed with an old 5,7 cm gun model 1896. Since this gun was ineffective against armoured target it was later replaced by a 5 cm gun. This could penetrate 50 mm at 1.000 m. making it one of the most powerfull AT guns in 1940.

Several years ago the casemates of Diefdijk came in the news. A firework factory, built around the northern casemate exploded, killing two men. Since then the casemate is visible again.