When you picture Venice, you probably think of the canal waterways, and the two columns that stand tall in the Piazza San Marco. The two columns were actually not built in Venice, but rather were the spoils of conquest in Constantinople. They were brought in by ship, and erected in the Piazza after much trial and error. It took the expert engineering of one Nicolo Barattieri to raise the two columns up. The space between them was later used as a marketplace and, during the 18th century, was the site of public executions. To this day, Venetians avoid walking between the pillars.
Sources: 203 Travel Challenges, The Architectural History of Venice by Deborah Howard (as cited on Wikipedia)