There are a lot of cannons. This was crucial to beating the Redcoats. The Water Battery in 1814 had two tiers of cannons, 26 in total. The British were unwilling to come within range of the cannons, and though this meant they avoided return fire, they were also too far away to successfully land their own bombs and rockets. So, eventually they retreated without ever having come in range of the Water Battery. The banner yet waved, just like in the song.