Short answers, pulled from the story.
The Winter Line was a series of German and Italian military fortifications built across Italy during World War II. It consisted of three interlocking defensive lines: the primary Gustav Line and two subsidiary lines, the Bernhardt Line and the Hitler Line. The system was constructed by Organisation Todt and commanded by Field Marshal Albert Kesselring.
The Winter Line effectively slowed the Allied advance for seven months, from December 1943 to June 1944. The Allies fought through all elements of the system from mid-November 1943 through June 1944.
The major battles at Monte Cassino and Anzio resulted in 98,000 Allied casualties and 60,000 Axis casualties. These two engagements were among the most costly of the Italian campaign.
The Gustav Line stretched from just north of where the Garigliano River meets the Tyrrhenian Sea in the west, through the Apennine Mountains, to the mouth of the Sangro River on the Adriatic coast in the east. It crossed the full width of the Italian Peninsula.
The Bernhardt Line was a subsidiary defensive position on the western side of the Apennines, standing to the southeast of the main Gustav positions. It merged into the Gustav Line northeast of Cassino. The assault on the Bernhardt Line began on the 1st of December, 1943, under Operation Raincoat.
Monte Cassino sat at the center of the Gustav Line where it crossed Highway 6, the main road north to Rome. The ancient Benedictine sanctuary above the town gave German defenders clear observation of any Allied forces advancing toward the Liri valley entrance, making the position extremely difficult to assault.