Built over a 13 year period by the father and son team, John and Washington Roebling. (with the invaluable support of the latter's wife Emily), the 6016-ft long Brooklyn Bridge was an incredible feat of engineering (costing the lives of many construction workers) and a quite breathtaking architectural statement. Supported by four steel cables, each 15 inches in diameter, strung across two immense neo-Gothic towers, the bridge was designed for horse-drawn traffic but was so well engineered that today it carries 6-lines of traffic between Manhattan and Brooklyn.
If you go:
Walk the bridge to Brooklyn, it takes about an hour and provides views you won't get anywhere else in the city.
Links:

Between Lower Manhattan & Brooklyn Heights
New York, New York 10038 USA
Mailing address:
NYC & Company
810 Seventh Ave., 3 Floor
New York, New York 10019 USA
Phone Numbers:
NYC & Company
+1 (212) 484-1200
+1 (212) 484-1200