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Catapults will return as an enemy siege engine in Diablo III. These enemies have not yet been seen in any screenshots or gameplay movies, or even in concept art, but they were announced in an @Diablo tweet in September 2010.

Demonic catapults never looked so cool. Now to decide what they’ll toss...

It's presumed that catapults will be found in the frozen northern Barbarian lands, were they were found in Diablo II. Not because they grow there, but because the only known siege in the game is of Bastion's Keep, a Barbarian stronghold players will battle through in Act Three.

Catapult Projectiles?[edit]

Upon the announcement of this type of monster, there was some speculation about what the catapults might throw. [1]

What does such a catapult throw? Quite a few things, historically. We often think of catapults or trebuchets hurling huge stones to break down walls or buildings, as seen in the Lord of the Rings films. But they were quite often used for psychological warfare or germ warfare, hurling the severed heads (and other parts) of slain defenders, the diseased, plague-ridden corpses of humans and animals were often hurled towards town wells, and they had other nasty tricks, like baskets full of poisonous snakes or scorpions. There’s a lot of historical precedent for the Diablo III team to use for inspiration.
I can easily imagine projectile versions of something like the Grotesque, with its posthumous cargo of Lamprey, or other bombs that split into various living demonic attackers. Bombs that spewed out Sand Wasps, for instance?