Desolator is best described as Defender without the little men. What is still there is the fast horizontally scrolling landscape, the green aliens (also known as the Gargoans), the radar and, to a certain extent, the rescue objectives. For lack of humans to rescue, you must instead capture the "Energiser"; an object which certain enemies will try to steal.
The game has, just like other Defender-like games, several types of enemies, from the rather dumb basic aliens, to stationary objects with high firepower, and quick hunters which might crash into you. Each level has a specific objective, though it is usually finished by killing everything that moves. As a last-ditch effort, you can activate the "desolator", which is a fancy name for a smart bomb device, killing everything on-screen.