Serenity Strull / Getty Images

(Credit: Serenity Strull / Getty Images)

It was the smallest, least-populated and shortest-lived colony in the US. But despite being virtually unheard of today, it helped shape the nation's birth 250 years ago.

The 125-year-old elevator wheezed to a halt somewhere above Philadelphia's skyline. When the door creaked open, I was inside the clock tower of the US's tallest municipal building, gazing down at " The Birthplace of America " from a 500ft (152m) observation deck atop City Hall.

From my glass perch, I could make out City Tavern, where the Founding Fathers plotted the American Revolution. Just west, I spotted Carpenters' Hall, where the Colonies united against the British at the First Continental Congress.…