Atari's Sentinel is one of the few Atari 2600 games that incorporates use of a light gun.
Your mission is to destroy deadly aliens hell-bent on invading the earth. You send the Sentinel, a large glowing orb that absorbs the energy of the enemies you shoot. Defend the Sentinel against enemy attacks, otherwise it will lose energy.
Sentinel is an arcade-style shooting game: you must shoot quickly and accurately at enemy power cells, ground and air attack vehicles, alien colonists and other obstacles to drain your enemy's energy and feed the Sentinel.
There are four increasingly difficult alien planets to conquer. At the end of each planet is an alien lord who must be destroyed to move on to the next planet. Destroy all the lords and face the supreme evil alien ruler.
Though technically a shooter, Sentinel is also a side-scrolling game. The Sentinel moves from left to right over the alien landscape. Below the action screen is a status screen that indicates the number of Sentinels remaining, your current power points, the distance left to the end of the level, and a super shots indicator. Super Shots destroy all enemies on the screen, and are activated by shooting the Sentinel itself.
In addition to shooting aliens, you can also shoot various icons that aid you in your quest. Icon abilities include extra Super Shots, Automatic shots that enable the Sentinel to automatically absorb energy from targets, Movement, which increases the Sentinel's speed, and Energy Capsules that recharge the Sentinel.