The Complete Roman Army, by Adrian Goldsworthy, explains the rule of war on sieges during the Roman reign.
The cost of resisting a siege
On page 197 the book says there was a general convention which eventually developed into a law on how defenders would be treated if Romans surrounded a city or town and laid on a siege.
Up to the moment a Roman battering ram made contact with the wall the defenders could surrender on favorable terms.
After the first swing of the ram all bets were off.
After that moment, all the men could expect to be slaughtered and all the women raped. Those surviving the initial rampage would be taken prisoner and sold as slaves.
Looting after a successful siege was extensive. Author says Josephus reports that after the destruction of Jerusalem in 70 A.D. there was so much gold and silver plundered and then distributed to the legionnaires that the price of gold was devalued all around the eastern provinces as soldiers returned home and started spending the money. A flood of gold into the market would of course bring down the price.
Cruelty after a siege
Even beyond the massive mayhem immediately after capturing a besieged city, the book also reports that excavations at multiple siege sites show corpses with signs of brutality, such as splits skulls, severed arms, or other indications of torture. One person was bound and then skewered with a spear. Yes, that means what you think it means.
Consider this comment, which I will quote because it provides such insight to the ancient world:
Perhaps a civil war created stronger passions and produced such atrocities, but it is important to remember that the ancient world was often an extremely brutal and unpleasant place.
Keep that in mind as you read of the supposedly unique barbarities of the Vikings or the ‘barbarians’ fighting the Romans.