Envahi is a side view fixed screen shoot 'em up where the player controls a helicopter and must defend a city that is being attacked by an invasion force. Next to the city is a large dam full of water that also has to be defended. The helicopter can be moved in all directions and shoots bullets upwards.

There are four different types of enemies to shoot down. Munchers move horizontally towards the dam and make holes in it upon reaching it. If the hole goes all the way through the water will flood the city. Nasties appear from a mothership at the top and move downwards until reaching a building. Slideys do the same but move faster and from side to side. Bully baliffs attempt to capture the helicopter and leave with it. Additionally the player has to watch out for rain drops since the helicopter is water soluble. A life is lost every time a building is captured, the helicopter crashes or the dam floods. Three lives are available at the start.