This is Space Invaders with a difference. You are a gopher and hordes of insects are invading your space. They are threatening to destroy your very own Kaktus (sic!) what you planted with your own two paws.
These aren't just any old desert insects. The bugs in Kaktus (note the mild pun here and feel free to laugh any time) release acid drops. These will kill you if you get hit by them, and will destroy the cactus if you are not careful.
Since your very raison d'etre is to protect the poor embattled cactus, it is not surprising that you should put your life on the line to save it. To ward off the pesky insect invaders, you must shoot through the hole at the top of your burrow and kill them.
Your exact weapon is not clear - and gophers are not generally noted for their marksmanship.
If the bugs do manage to break through the ground above your head, you can shoot them through that break in the ground.
If you know how to win at Space Invaders, figuring out Kaktus should be no great problem. Like the shields in Space Invaders, the ground above your head can serve as an alternative place to fire through. It can also serve as a protection from the bugs blasting down on you.
You can move out of the hole and face the bugs from above ground. But, unless you are highly dexterous in keeping away from the bugs, this is sheer video suicide.
If you stay above ground for too long, you run the chance of having a mole cover your hole over with dirt trapping you in the land of the small, stinging beasts. So the best strategy remains shooting from below the hole. Take advantage of any holes that are blown in the ground by using them as portals through which to blast the acid-dropping critters.
You get three lives. So you can afford to take a few chances on the first one, and perhaps pop up to the ground to rack up a few points. Then become more conservative in your play with the other two lives.
A nice variation on the theme. The concept of acid-dropping insects, armed gophers and marauding moles must be ranked among the more inventive introduced for games on the Vic 20. You will need an expanded Vic to run the game and least 8K. It will support a joystick.