You know how in the Middle Ages defenders of a sieged castle supposedly poured boiling oil on the attackers to drive them back?
You know how historians are pretty sure that's a myth?
A lot of people still don't seem to know, especially in tabletop gaming, so let me give you a few reasons why historians are probably right:
Until well into the Industrial Revolution, (lamp) oil was expensive. You couldn't afford to waste it like that, assuming you could even get a cauldron's worth of the stuff in one place.
Firewood is a lot of work to gather, too. You can't do that work while your castle is under siege, either, and you need the fuel for other things. Doubly so as...
...Oil has enormous thermal inertia. You'd need a whole lot of wood to make it boil or almost, not to mention a lot of time. The battle may well be over by the time you can get your cauldron ready.
Even if you can get it ready in advance, or the battle is long, that stuff would be incredibly dangerous to handle. The crew is more likely to die horribly in an accident than pour it on the enemy.
Assuming they manage to safely get the cauldron to a murder hole, the people below are equally likely to be their own side, especially if the battle has made it inside the gatehouse by that point.
Last but not least, how many attackers are you going to get by splashing down the content of a, say, 20-liter cauldron? Fine, 40 liters. 80. A barrel. You could have spent all that time shooting hundreds of arrows instead. Flaming arrows. While using a lot less wood and oil. No. Just no.
Wait. How about water then? Maybe it was hot water they poured down instead?
Admittely, that makes marginally more sense. But just barely:
Water is a lot easier to get ahold of than oil. But fresh water is still precious in a castle under siege. Still no.
Water takes a lot less fuel and time to heat up, and can still cause crippling damage even before it nears the boiling point. It's still not enough of a difference to make it practical.
You'd have to painstakingly carry it by the bucket first out of the well, somewhere below the castle, then up the ramparts. Among fighters engaged in a pitched battle, and other hazards. That's a long way at the best of times.
All right, wise guy. What would you give the rearguard of a castle to help out with the fight?
First of all, people in the rearguard would have their hands full in support roles, such as passing arrows and bandaging the wounded. That said, rocks. Rocks are readily found, don't take much space, and can easily crack heads when thrown by hand, let alone with a sling. Something a lot of people in the Middle Ages learned how to use, because it took relatively little training and had many applications, while being cheap and quick to make.
Speaking of which, you do know that medieval weapons were in fact pretty damn slow and expensive to make, right? But that's a story for another time.