Kline, Otis Adelbert: Maza of the Moon. v1, 27 Feb 2008
Classic "pulp" SF, very much in the style of Edgar Rice Burroughs.
Following the impact of a projectile fired at the Moon from the Earth, the Earth is attacked by a previously-unknown lunar civilization.
Everything you'd expect from the genre; evil villains, beautiful princesses, hideous monsters, etc. Great stuff!