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,