It was the site where St. Benedict put his first monastery in the 6th Century, because it was an excellent defensive position commanding the road to Rome. I believe the original buildings were sacked and destroyed by Saracen pirates a couple hundred years later.
Here's an interesting site.
http://www.sspxafrica.com/documents/2005_JanFeb/Bombing_of_Montecassino.htm
According to this 11 monks stayed in the abbey and only one died but he died of Typhoid.