Keef,
I was once contacted by a fellow who had the job of wiring the Old Executive Office building for a local area network back in the early days of networking. While not built in the 16th century, it was built in the latter half of the 19th century as the State, War and Navy Building*. That's "Old" by Washington standards. Very solid with 12"-thick masonry interior walls. And it was a National Historical Landmark so the Preservationist had to approve everything.
* Yes, it originally housed the entire State Dept (Foreign Office), War Dept (Army) and Navy. Now, it doesn't even hold the President's Executive Office and a much larger building has been constructed up the street. The building was referred to by Mark Twain as "the ugliest building in America." Harry Truman called it "the greatest monstrosity in America."
Old Executive Office Building - Wikipedia, the free encyclopedia