"caused by a mortar-shell attack on the villa two months earlier"

A mortar is a weapon made up of a tube, into which an explosive shell is loaded. During World War II mortars were used to fire across enemy lines.

In Fiesole, mortar shells were launched at the villas being used as strongholds for German forces, causing a great deal of damage. At the Villa le Balze, as described in The English Patient, mortar shells and machine gun fire caused a lot of damage:

"In August 1944, advancing forces believed German troops to be defending the Villa on the night of their attack on southern slopes of Fiesole, and light artillery was brought to bear. Artillery shells destroyed a large part of the Villino roof, a nearby garden wall, and on the main house caused major damage to the south and east loggias. ... One shell even crashed through the roof and upper floor and lodged itself in the library, but, miraculously, it did not explode."

From History of the Villa, on the Georgetown University website.