From Pope Pius XII’s Encyclical Fulgens Radiatur:
Trusting in God and relying on His ever present help, he went south and arrived at a fort “called Cassino situated on the side of a high mountain …; on this stood an old temple where Apollo was worshipped by the foolish country people, according to the custom of the ancient heathens. Around it likewise grew groves, in which even till that time the mad multitude of infidels used to offer their idolatrous sacrifices. The man of God coming to that place broke the idol, overthrew the altar, burned the groves, and of the temple of Apollo made a chapel of St. Martin. Where the profane altar had stood he built a chapel of St. John; and by continual preaching he converted many of the people thereabout”.