How did the Romans conquer the world sustainably? Sustainability Part 2

Why am I holding a 2000-year-old nail in my hand? And how did it get to me?

The Romans were masters of strategy, so it is not surprising that they reused materials in their warfare. But what role did this play in their warfare? How important was the nail, for example, in the siege and capture of foreign territories? And why were they sometimes buried in large quantities?

Sustainability: a term usually associated with the beginning of the 21st century. But what was it really like? What do the Romans have to do with this and what role did it play for the Middle Ages?

Iron ore was an important resource in the Middle Ages, so it is not surprising that it was better not to leave it in the hands of the enemy. But what was iron used for? Why was it so important in warfare? And what did they do with it when they no longer needed it?