“shofar.” The ram’s horn trumpet, not the metal trumpet. In the Day of Yahweh there will be wars and conquests, so there will be trumpet blasts and battle cries (cp. Matt. 24:6; Rev. 6:2).
“the high corner towers.” The corner towers of a walled city were the most heavily fortified places in the city wall, so if the attack is against those, the defenders have no chance of victory.