Crossfire

In this adaptation of the arcade game Targ your job is to defend your city from marauding alien beasts. You can simultaneously maneuver your craft and shoot in four directions while patrolling the regular grid of streets - the enemy has surrounded your city block, and attacks are coming in from all sides as the aliens swarm in with all guns blazing. To survive, you'll have to blast your way out by darting in and out of the line of fire and taking down the assailants with your own volleys: once they're all eliminated, you go forth to the next level, to face yet another wave.

Ammunition is limited, and from time to time you'll have to restock by picking up a fresh supply. Every level contains four bonus crystals: as time goes by, they emerge one-by-one to be picked up, and each is worth twice as much as the last. The game starts you off with three ships, though you get a fresh one for every 5000 points you score.