Complex sentence: Fort Eben Emael, which measured had been built during the 1930s, and completed by 1935, by blasting the required space out of marl and possessed walls and roofs composed of thick reinforced concrete, as well as four retractable casemates and sixty-four strongpoints.
Split into simpler sentences: Fort Eben Emael, which measured, had been built during the 1930s, and completed by 1935, by blasting the required space out of marl. It possessed walls and roofs composed of thick reinforced concrete, as well as four retractable casemates and sixty-four strongpoints.