Complex sentence: Organisation Todt (OT), which had been formed in 1933 designed the Siegfried Line during the prewar years along the Franco-German border, was the chief engineering group responsible for the design and construction of the wall's major gun emplacements and fortifications.
Split into simpler sentences: Organisation Todt (OT), formed in 1933, had designed the Siegfried Line during the prewar years along the Franco-German border. OT was the chief engineering group responsible for the design and construction of the wall's major gun emplacements and fortifications.