To see if the prototype could be fun I started by making a paper prototype to test the gameplay with people. During my tests I concluded that the gameplay is fun, but very hard to paper prototype, because all players need to control things on the spaceship and it is difficult to keep the paper in the right spot while multiple people are using the same piece of paper. We also needed someone to control the enemies so we needed more players than the actual game needed to test the paper prototype.
The paper prototype showed that the gameplay and cooperation it creates can be fun.
After testing this with some players I started on a digital prototype to test the movement of 4 different players.