Rule 310.7 · Battles
If a Siege battle’s defense is 0 and it isn’t the source of an ability which has triggered but not yet left the stack, it’s put into its owner’s graveyard. (This is a state-based action. See rule 704.)
Ask the judge Pro
The life counter has an AI judge that answers your play citing the exact rules. It is part of Gatewatch Pro; the rules, Oracle text and rulings stay free.