The Gustav Line was the main German defensive line that spanned from the Tyrrhenian Sea to the Adriatic Sea. The Gustav Line ran along the Garigiliano and Rapido rivers on the west and on the Sangro river on the east side of the Italian peninsula. The line was defended by 15 German divisions fortified with small arms, artillery, pill boxes, machine gun emplacements, minefields and barbed wire. The German divisions had retreated to and fortified this line after the the Italian surrendered to the Allies on 8th September 1943. In order to reach Rome, the Allies had to break this line.
The lynchpin of the Gustav Line was Monte Cassino." identified by the Germans and Allies as key terrain because of the outstanding observation it provided over the entrance to the Liri Valley. From Monte Cassino, one can see every road and river crossing at the mouth of the Liri Valley. Increasing the complexity of the Monte Cassino terrain was the Benedictine monastery built on top of it. The Rapido River formed part of the Gustav Line, acting as a natural moat, protecting Monte Cassino.