Rats
[ranking: 16]
Rats weren't just a good way to spread disease; they also got into the enemy's food stores and ravenously devoured precious grain and vegetables. While not easy to catch or deliver (alive) via catapult, rats introduced in sufficient numbers could drastically shorten the length of a siege.
Fire
[ranking: 8]
Fire has long been used as an implement of siege warfare, and it's seen a lot of applications. Infiltrators (sappers) used fire to burn the wooden supports out from underneath walls, while others would shoot flaming arrows and projectiles into the fortification. The smoke in itself could be a nasty weapon; armies would often load burning projectiles with noxious substances like chicken feces and poison ivy, which would turn the smoke cloud into a kind of stinky tear gas.
Siege Tower
[ranking: 3]
Sometimes also referred to by the generic "siege engine," towers were as much about psychological battle as practical warfare. Siege towers were basically glorified ladders meant to be wheeled up to enemy walls so invaders could quickly scale them and invade en masse. The sight of a line of massive siege towers appearing over the horizon was often enough to break a defending commander's will before the siege even began.
Cannons
[ranking: 4]
Cannons and big guns completely changed the rules of siege warfare almost overnight, altering the course of history almost as quickly. In 1453, the Ottoman Empire used its advanced casting and metal-working technology to produce ridiculous behemoths like the one pictured.
Mehmet II used guns like this to break down the walls of the Christian Byzantine city of Constantinople, which were thought to be unbreakable for more than a thousand years.
Ballista
[ranking: 1]
Ballistas were effectively giant crossbows, working on the same torsion principles as onagers. The big difference was that ballistas were meant to fire straight ahead at enemy walls instead of up and over them. Loaded with stone "cannon balls" as often as they were with the iconic "arrow" bolt, ballistas specialized in hammering enemy walls to pieces - but they could just as easily be fired into advancing soldiers.
Battering Ram
[ranking: 2]
Since a Lord of the Rings reference is pretty much inevitable on this list: Yes, Grond, Hammer of the Underworld was kind of a real thing. Granted, most invading armies didn't consist of orcs, and most rams weren't intricately carved, flaming wolf heads. Most were like the one pictured - logs suspended on ropes and covered by a roof to protect from arrow fire.
Some had flat faces, some were pointed and some had metal spears depending on whether they were used to batter down walls or wooden gates. And yes, some (like those used by the ever-stylish Romans) even had intricately molded, metal ram's heads.
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