The definition of a challenging game.
In the game, the player takes control of an Assault Suit, which is a large battle robot, and guides it through several stages in locales ranging from the surface of Ganymede to Earth's orbit to the surface of Earth itself. The stages are very challenging even on the "Normal" difficulty setting, and nearly impossible on the higher ones. Most of the stages involve shooting your way to some sort of boss, and destroying the boss to advance to the next area. A couple of the stages are defensive in nature, for example, holding off the enemy long enough for your shuttle to make entry into Earth's atmosphere. This occasional departure from the regular kill-the-boss formula keeps the game interesting.
What makes the game so challenging is the sheer number of enemies that are thrown at you. There is a literally endless amount of enemy assault suits that will spawn in each stage, and they will keep spawning until you either reach the boss or finish your mission. On the "Hard" setting, enemies are much harder to kill, and on "Hyper", which is the highest setting, they are not only harder to kill but their weapons are far more powerful. The only thing you can count on is that they will always spawn on the right side of the screen. Occasionally, friendly (and near worthless) units will spawn from the left side of the screen and try to help you out as you make your way to the end of a level.
The silver lining to having an infinite number of enemies shooting at you is how many points you get for killing them. In most games, your score is rather meaningless, but not in Target Earth. Here, your score decides what equipment you get to use in the next stage. If the player has a high score at the end of a level, then many new guns, armor, and other equipment will be available to use in the next level. Conversely, if you didn't do too well in the last stage, not much will be available to you for the next one. Also, there is only a limited number of items you can take with you to use in the next mission, so you must choose wisely from your stash of supplies. This is where the depth comes in, and it allows for some strategy to be used in the game as you make your way through. Bonus points are also given for completing special objectives in each stage, such as rescuing your allies in stage 2.
The story of the game is a little strange, a space colonization gone wrong has resulted in an army of angry, evil cyborgs, and they have come back from space to destroy the world that forgot about them. The main character's girlfriend serves on the same assault suit team as his own, an idea that was revisited in Cybernator, the next game in the series. It's not a complex plot, but it serves its purpose.
The graphics in the game are extremely well done. Every landscape and sprite is detailed and well drawn, and most of the assault suits you see have nice walking animations. A portrait of the characters will appear when they speak as well, which was a nice touch. Explosions are also well done.
The music and sound were the low point of this game, as there are only a few tracks to listen to and the sound effects are merely average. However, they still manage to generate a pretty appropriate atmosphere as you battle your way through the game.
Target Earth was a great addition to the Genesis library, and I advise anyone who still has a working console to find it and play it. It's a pretty tough game, but you won't regret it.