A Mighty Fortress is the fourth book in the Safehold science fiction novel series by David Weber and published by Tor Books on April 13, 2010. It debuted at #9 on the New York Times hardcover fiction best seller list, following in the steps of previous titles in the series which also debuted on the list. The book dropped to #24 in the second week, then to #29 in week three before dropping off the list, for a total of three weeks on the list. Preview chapters of the book were published before the book was released.