The Hebrew verb tsur means to besiege, to confine, to cramp, or to press/distress. It is the word for laying siege to a city — surrounding it, cutting off its supply, pressing it until it must surrender. It is related to but distinct from the noun tsur (rock, H6697).
While God is the Rock (tsur, H6697) who defends, the verb tsur (to besiege) appears in contexts of both divine judgment and human warfare. When God's people are disobedient, he permits enemies to tsur them (Deuteronomy 28:52: 'They will lay siege to all the cities throughout your land'). But God also tsurs the enemies of his people (Isaiah 29:3: 'I will encamp against you... I will besiege you'). The verb creates a striking theological image: the God who is your Rock-fortress can also become the siege-master pressing against those who oppose him. No city of rebellion stands forever against divine pressure.