Built in 1901 as an expansion onto the Washington Aqueduct’s receiving reservoir, the castle gatehouse is a rare example of architecture based on a government logo, the so-called “Corps Castle” insignia.

The castle stands atop the entrance to a four-mile-long tunnel that connects with an early bit of public health infrastructure: a Slow Sand Filtration facility on the other side of Washington, D.C.

The castle and tunnel were built as a part of an effort to reduce annual typhoid epidemics that plagued Washington during summertime. Prior to their construction, all tap water in the city was unfiltered, muddy, and laced with bacteria.

Inside the castle, Aqueduct employees can monitor the stream in a little observation pool before it enters the tunnel. At this preliminary stage in the filtration process it’s not uncommon to see marine life like fish and even eels. There’s also a large iron sluice gate that can be dropped down to cut off the flow if maintenance is necessary.

The castle gatehouse is a local landmark that’s loved by locals, even if they aren’t privy to its technical purpose.