I know that people used boiling pitch to drop on unlucky attackers trying to get in a castle. However, some time ago I heard somewhere that they also used lead.
This sounds particularly nasty, and I don’t see why it wouldn’t be feasable using medieval technology to gather up lead and melt it down. Lead has a relatively low boiling point, and it is very dense, which works to its advantage when poured as a liquid.
However, I can imagine that this stuff would be incredibly hazardous to lug around. Didn’t castles have ‘spouts’ and channels that hot liquids like oil and lead could be poured at the attackers?
Lead was used as a roofing material so in a time of crisis would have been at hand. All the sites I have seen suggests that it was used in battle. One of its main advantages was that with a high heat capacity it kept its heat longer, could be heated hotter and so could cause more damage. As to its hazards, its toxicity is nothing compared to what the attackers would do to you if they broke in
On top of which, they were unaware of any toxicity at that time. It’s only recently that we’ve realized that lead plumbing and lead based pigments are bad ideas (lead oxide happens to make a very lovely white pottery glaze). The only danger they would have thought of in handling it would be the physical dangers of handling any extremely hot liquid.