Top-down shooter / Fake 3D

My objective is to make something like:

If your character is not on the same floor, the bullet shouldn't hit the enemy and you can jump up and down between floors freely.

Here is a game that uses this mechanic: cross-code.com/en/play

Do you guys know an effective way to do that ?

Thank you for reading!

You could use layers (Is on layer), or an instance-variable (via Families) that indicates the floor a character is on. Just add this extra condition for any collision testing.