The description is that of a siege, such as the one Jerusalem suffered in 597 BC (2 Kings 24:10), and again disastrously in 586 BC (2 Kings 25:1). The day of the Lord will be a day of battle against even the most secure positions. This was one aspect of the predicted curses on covenant breakers (Deuteronomy 28:52).1
16 a day of trumpet blast and battle cry against the fortified cities and against the lofty battlements.