Different types of aliens move differently, so you'll want to memorize their behavior.

The yellow aliens move forward regardless of the danger. So, if you let them walk towards a cliff or a monster, they will simply die.

The blue aliens are a bit more intelligent and are able to turn around once they reach a cliff. Of course, they also get eaten by monsters, if they encounter them.

The red aliens are the most valiant ones. They are able to slay monsters and clear the path for all others, so send them in first.

Lasers can be both essential and dangerous. They can power the satellite which calls the spacecraft, but they can also pierce a hole in your aliens if you're not careful.

The satellite is also powered by buttons, which your characters need to press or hold. Keep in mind that sometimes you'll need to press a number of them at the same time to get the satellite working.

There is also the game's undo button to save you from the trouble of restarting the level because of one or two silly mistakes. Use it abundantly and learn from your mistakes.