Threshold

Developer's blurb Threshold, by Warren Schwader and Ken Williams, features fast smooth animation, hi-res graphics, and more challenge than you'll find in any other arcade game on the market. Threshold is an arcade game with alien attackers galore. In fact, there are more monsters out there than we expect you'll ever see. Threshold was designed to be an arcade game that you won't get bored of, and that means a constantly changing game with a graduated skill level, but more than that, Threshold means constant fun.