I was wondering if there's any evidence of such a contraption in the Middle Ages? I'm imagining this as a cousin to the catapult; instead of catapulting rocks on attackers, they simply drop rocks on them. My thought is that it's designed for when the attackers get too close to the fortification?
It would be more efficient b/c it allows for larger rocks that are not easily liftable.
What you ask for sounds much like a machicolation. Boiling oil or water were more traditional than stones, but stones could be dropped as well. Within a gateway, this would be called a murder hole.
There are reports of artificial avalanches to defend hill forts, but those probably do not meet your definition of 'contraption' ... simply an unstable pile of rocks that could be rolled downwards.
Please note that your question sounds a little bit too basic for this board. People are supposed to check the literature to refine their question. If you weren't a first-time poster, I might have voted to close.
Not really, for two reasons:
1- Gravity works fine
The idea is to defeat attackers when they are the most vulnerable. Which is when they try to storm the castle. Why set up complicated machinery if you can simply drop rocks, boiling water or oil from a murder hole?
2- The foundation couldn't handle it
A catapult has a severe impact on the place where they stand. The force used to hurl a rock has to go somewhere. That's the reason why an onager was called an onager: it kicked like a mule.
First of all, there was rarely enough room on a wall to place such a device. Second, supposing you would be able to place one on a turret or large enough space: the "back blast" (for lack of a better word) could destroy the place it stood.
If you placed it behind the wall on ground level, you would have severe problems aiming the thing. Indirect fire wasn't invented yet.
There were some cities and castles that could accommodate catapults on their ramparts, for example Constantinople. But they were the exception, not the rule.
