Great port of an old Universal arcade game, which may have been the first to feature ladder-climbing! This is kind of like a side-view of an arcade game from Japan called "Heiankyo Alien" (that didn't have ladders but did feature burying aliens). With one button you dig a hole through the floor, and with the other you bury the hole. An alien can fall into the hole and this is your chance to fill it with the bury button. Dig or bury, it takes 3 hits of the button. The red aliens only take one buried hole to kill, and this knocks them down a floor. If your timing is right you can take out another monster this way and get extra points! Later in the game you'll encounter enemies that must drop more than one floor (you need to dig a shaft of multiple holes below each other-- and they have to be dug DIRECTLY below each other or it won't work). The green alien must fall at least two floors, and the blue alien must fall at least 3. But it is SO much easier just to get the timing right and drop an alien on another-- no matter which color alien it is they both die! Letting an alien dig its way out of the hole can make it mutate to the next strength of alien, as can dropping it less floors than required. You can drop an alien MORE floors than required, for bonus points! Great port of an underappreciated arcade game by Universal, known later for their Mr. Do series.
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