“I set watchmen over you.” These “watchmen” were the prophets who warned them of the coming disaster if they would not repent and change (cp. Ezek. 3:16-21).
“shofar.” The ram’s horn trumpet, not the metal trumpet. The shofar warned of danger and battle and people did well to pay attention to it and prepare for potential disaster, but the people of Judah refused to pay attention, and as a result were conquered and deported by Babylon.