Alien Syndrome is a shoot'em up game released by Sega in 1987 for arcade platform, and the DOS port was released in 1989.
For the DOS port, it uses 256-color VGA graphics, but strangely it does not support any soundcards beside PC Speaker. However, Sega at least try to emulate digitized sound using PC Speaker, but their technique is not as good as the Access one, like Means Street.
Alien Syndrome allows one player and two players mode. The main character that we control are two soldiers named Ricky and Mary, who must fight their way through large eight-way scrolling levels rescuing their comrades who are being held by aliens.
After they have rescued a certain number of hostages the exit opens and they can pass through it in order to fight the level boss. If the level boss is defeated, they move onto the next stage.
On the firepower, we start with a simple gun only. There are a number of pick-ups that we can collect in the wall of the level, which includes laser gun, flame thrower and other fancy weapons.
Aline Syndrome is a nice arcade game, but it requires some practice. Due to some programming weaknesses, the game will occasionally slow down, especially when the screen is loaded with lots of monsters.
You can download Alien Syndrome here.