A siege engine is a mechanical machine that is designed to hurl projectiles, typically at the walls of an enemies castle or fortifications. They are called "Siege" engines because their most common use was in aiding the efforts of a siege where the enemy was holed up within the walls of its fortification and waiting things out. The siege engine was used to tear down the walls, breaching them, so that military forces could enter the fortification.
So let's take a look at some of the different types of siege engines
There are a few different types and there are of course quite a few variations on each of them. This is to be expected when you have a couple of thousand years of development. I will give you a good overview of the major types and some of the major variations.
The goal of a siege engine is either to destroy fortifications or somehow circumvent their effectiveness and there are four major types: The Catapult, The trebuchet, the Siege tower and the battering ram.
The Catapult: Is a wooden machine that is designed to hurl projectiles, often stones. The drawing below gives you a good idea of what they look like although you probably already know that. There are variations of catapults and the mangonel style shown uses twisted ropes to get the force for hurling. And the mangonel has a bucket for the projectile. Another variation of this would be the Onager which was very similar to the mangonel in that it uses twisted rope for force. But instead of a bucket it had a rope and a sling to hold the projectile.
The Trebuchet - This is a much more powerful type of siege engine and it was first developed in the late middle ages. It used the power of gravity to hurl projectiles. It is a much simpler design than the catapult because you didn't need a complex and dangerous wrapping of rope for the force. These siege engines could get to be very large in size and they could hurl very large and heavy projectiles.
Siege Tower - A Siege tower is a tower that is rolled up to the walls of a castle or fortification. The goal of the tower is to allow soldiers to go over the wall and into the fortification. You can see there is a ramp that is dropped. This is the ramp they use to go over the wall.
Battering Ram -
You probably have a good image in your mind as to what a battering ram is and you would be correct. It is some kind of a long log or piece of lumber that was used to break down gates, doorways and even walls. Sometimes it was carried by men and other times it was mounted on some kind of a wheeled platform so it could be rolled up to the walls. Battering rams could also be part of a complex building on wheels that offered overhead protection to the attackers. The roof would protect them from arrows or other things being dropped on them from above.
Want to see a big Trebuchet in action? I built one ten feet in size and it hurls six pound rocks. The video is on my youtube channel right here: The Big Trebuchet
Want to make your own catapult or trebuchet? I have a whole lot of tutorials on them right here. Make popsicle stick sized, table top sized or bigger! How to Make Catapults
Amazon.com has Catapults and more