HOW IT WORKS:
The collisions in the SHMUP Creator are a little weird: they use a mix of 3D collision and 2.5D collisions. What are 2.5D collisions?
All the gameplay objects, the player, the bullets, the enemies, and the items use 2.5 collisions. It means that the collisions boxes are transformed in 2D (like projected on the screen).
The result is that all the collisions are on the same plane. For instance, if you have a helicopter flying and shooting bullets, and the bullets, on the screen, are at the same position as a tank on the ground, the tank will explode. Even if in real life the bullet is on top of it.
We choose this trick to help unify the different types of games you can make.