Look, evidence that there's something inside the fort! These guns are along the northwestern wall. There are seven of them there, and I couldn't tell you their size or calibre. The only other gun mounted at Fort Independence is a 15" Rodman Gun, on the southeast bastion.

Here's what the northwest wall looks like from above. This would have been the original configuration for the fort's guns when it was first built. Today, there are mounts for 14 or 15 more modern guns (most likely those good ol' Rodmans) along Fort Independence's walls and bastions.