I think you're wrong about the thousands of civilian casualties. I believe there were only a few monks resident in the abbey at the time and few were killed. Bombing the place turned out to be counterproductive as the Germans found the resulting rubble made much more effective cover from the US infantry assault than the intact buildings would have. That's what I've read anyway.
I believe it was on a mountaintop. Not exactly a heavily populated residential area. The real significace is the fact that it was a holy Christian site that we bombed mercilessly because that's where the enemy was. If I'm not mistaken, we got permission for it.
I've got a few old WWII documentaries, Frank Capra type, the Battle of Cassino was a bloody one.
Most people dont know, almost 50% of all the deaths of US troops in Europe in WWII happened in Italy.