A guy flew his drone over the abandoned Fort Carroll near the Key bridge and the footage is amazing.
The fort was built in the mid 1800's in a plan to build more defenses to protect the port of Baltimore since at the time, the only major defensive protection was Fort McHenry.
The construction of the fort was halted at the outbreak of the Civil War, and you can see in the video how some of the walls are taller than the others. The fort was armed with cannons for a short period of time, but armaments were moved during World War I and the fort was officially abandoned in 1923.
It's now an animal sanctuary.
Source: Wikipedia