"Caveman brain" => lmao ! The small bullets are actually downgrades. The idea is for the player to only shot the zombies corresponding to the upgrades. The spanning in wall is unfortunate indeed, there is a check to not make them spawn outside a defined area and another to not make them spawn on top of another, I wonder how they still achieve that :(. Thanks you for your feedback, I really appreciate.
EDIT: Oh I know, when they flee the might stuck on the wall since I update their position and not their velocity... silly me.