Complex sentence: Every three years the college also holds a ball, usually off site due to the problem of securing the college's perimeter sufficiently for insurance purposes.The most recent off-site ball was held was February 9,2008 at Heythrop Park.
Split into simpler sentences: Every three years the college also holds a ball, usually off site due to the problem of securing the college's perimeter sufficiently for insurance purposes. Until recently, the last ball held was February 9,2008 at Heythrop Park.