The first one that comes to mind is a cannon or something that shoots projectiles. Also, you could make something that attracts zombies, like a brain decoy, and use it to bring them to a trap or just to gather them in a point so you can take a break. That would be a "passive" structure.
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Brain decoy is a good idea. In the zombie wandering audio sfx list, there's a zombie that says "cerveaaaauuuuu" like a zombie. The word "cerveau" stands for "brain" in French haha. My son already drew a fireball sfx, but I think that is for another playable character that I couldn't integrate in the first iteration of the game :) Thanks for your ideas !