The approach to Baltimore by water was guarded by Fort McHenry by obstructions sunk in the channel of the river and by two heavily constructed batteries between Fort McHenry and the city.
Here however the British met with an unexpected repulse. The bombardment of Fort McHenry began at sunrise, on the 13th [September], and continued throughout that day and the succeeding night, though without reducing the fortress. Under cover of the darkness, several rocket vessels and barges ascended past Fort McHenry, but being detected were received with a heavy cannonade. [Source]