Complex sentence: Block 3 is essentially the same as the large artillery casemates built by CORF for the main Maginot line in northeastern France, with 75mm guns on the upper level and 81mm mortars firing upwards out of the pit in front of the guns.
Split into simpler sentences: Block 3 is essentially the same as the large artillery casemates built by CORF for the main Maginot line in northeastern France. With 75mm guns on the upper level and 81mm mortars firing upwards out of the pit in front of the guns.