Most games of the time were based around the idea of destroying wave after wave of aliens without any real need for thought. For obvious commercial purposes, we needed to incorporate this kind of action but wanted to add a puzzle element to the game without giving it an educational edge - a game that was reasonably difficult but that relied on intense individual skills and thought behind actions made. Many games of the time were purely about destroying waves of alien spacecraft and that was it - we favoured a two pronged approach without overcomplicating the gameplay so it could maintain longevity.
The game centred around a moonbase, largely plagiarized from Star Wars which displayed a number. There were six sets of different aliens: the first, which varied through six levels, appeared as random waves sent out from the moonbase which needed to be destroyed. The others were numbered between one and ten which had to be repelled back into the base to lower the total. The figure had to match exactly to clear the level otherwise you lose two lives or just the one if you had one remaining
The moonbase has a completely random number between one and ninety-nine which is displayed on the front. There are six different waves of ships: one that is numbered individually and needed to be repelled back into the moonbase by bouncing them off the ship as opposed to destroying them, sending them back into the moonbase to get the figure down. The others were purely out to destroy your ship.
The longer the level went on, the more frantic the game became. Not only that, if the Aliens managed to get past you, they formed a line at the bottom of the screen. The numbered aliens would fire lasers back towards the ship to try and increase the number again and take you out as well, as well as the normal ones gathered there trying to kill you. The longer you dithered, the more intense the gameplay came.
We studied a great deal of footage on synchronized swimming and flight patterns of wildlife to gain inspiration as to how we can vary the imagery of the oncoming spacecraft. Galaga was a good example at the time of how you can make the gameplay graphically enjoyable and a pleasure to watch.
We admired Astro Blaster, Phoenix and other games of the ilk but we wanted people to feel they were having an experience as well as playing a standard video game and neutrals to appreciate it. Our main priority was the atmosphere, effects and playability which was the key to keep people playing in our opinion.